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GPO Telephone 300
General information

This page contains some general information about the 300-series telephone sets, introduced in the 1930s by the British General Post Office (GPO) and made by a variety of manufacturers. Some of these sets, in particular the 328, 394 and 396 models, were used during and after World War II (WWII) with a so-called Frequency Changer — an (insecure) inversion-based speech scrambler.

The GPO 300-series was originally developed in cooperation with Ericsson UK and is based on the Ericsson N1015, N1016, N1017, N1374 and N1376 models [4], albeit with a British GPO No. 164 handset. This is the same handset that came with the earlier Telephone No. 162 (the Pyramid).

There were models for use with Local Battery (LB), Central Battery (CB) and Central Battery Sig­nalling (CBS) systems. Furthermore they could be configured with or without a dial, with or without a bell, and with up to three push-buttons just above the dial, for a variety of special functions.
Tranasparent series 300 TMC  telephone set

During WWII, a speech privacy system was introduced under the name Frequency Changer. It was based on the inversion of the audio frequency spectrum, and is therefore also known as a voice scrambler. Telephone sets that were used with the scrambler were quite different from regular ones, but were based on the chassis of the 300-series, in particular the 328, 394 and 396.

In fact, when there were supply shortages during the war, regular 300-series phones were often converted for use with the scrambler, either at the GPO workshop or by technicians in the field.

Original scrambler phones had a 4-digit model number, prefixed with the letters 'SA'. In most cases, they were fitted with green handsets, to discriminate their lines from regular unprotected ones. They were made of green bakelite, as with the SA-5063/1 shown in the image on the right, but could also be regular black bakelite ones that were painted green as a gap-fill solution.
SA 5063/1 voice terminal

Below is some general information about 300-series telephone sets, that we came across during our research, in particular when restoring the scrambler phones in our collection. More detailed information is available from various websites, some of which are listed at the bottom of this page. For further information about telephone sets that were used with the Frequency Changer, please refer to our page about the WWII Scrambler Phone (Secraphone).

TMC demonstration set in transparent acrylic enclosure

Known models
A few examples of phones in the 300-series:

  • 332
    Basic unit with dial, bell and handset (no buttons)
  • 330
    Version for private branch exchange (recall button) 1
  • 312
    Version for Shared Service installations (party line)
  • 332 CB
    Version without dial (and sometimes without drawer)
  • 394 LB
    2 or 3-button version for CB systems
  • 396 LB
    2 or 3-button version for CBS systems
A few example of telephone sets that were derived from the 300-series:

  1. The recall button connects earth (ground) to the telephone loop.

Standard terminal block
The drawing below shows the standard terminal block that is present on all 300-series GPO phones. It is mounted at the bottom of the metal chassis and is normally used for connection of the line and the handset. All components of the telephone, such as the transformer, the bell, the capacitor, the dial and the hook switch are connected to this terminal block. Note that on Secra­phone voice terminals, additional wiring is present via the 9K terminal block. Here is the layout:

Connection block T as seen from the bottom of the phone

For a standard telephone set, the line is usually connected at 1 (A) and 9 (B), and the handset is connected at 4 (M), 5 (MR) and 6 (R). There should be shorting strips between 1 and 2, between 8 and 9 and between 10, 11 and 12, as indicated by the red lines in the drawing above. If you want to convert a telephone set for use on 'modern' analogue systems, check out this website [6].

 How to convert a telephone for use on modern networks (off-site)

Interior bottom view
Connection block (T) seen from the bottom of the phone
Key Unit 303/A (9K switch pack)

No. 303/A key unit   9K
The 394/396 telephone, that is used as the voice terminal of the Scrambler, has two or three switches at the top of the bakelite case. These buttons directly operate the 303A key unit (also known as a 9K switch pack) which consists of two 4-pole make-before-break switches (4K) and one single-pole make-before-break switch (1K) mounted in between the other two. For other types of key units, check out Robert Freshwater's excellent website [3]. The layout of the 9K:

Connection block 9K as seen from the rear of the phone

Each of the contacts of this terminal block, with the exception of contact 28, is wired to one of the contacts of the 9 individual switches. Contact 28 is not connected (nc). When looking from the front of the telephone, the three buttons are identified (from left to right) as A, B and C. Pressing a button, automatically releases the other buttons. Depending on the configuration, the keys are latching or momentarily. In our case, only the rightmost button (C) latches when it is pressed. On some phones, the buttons are also released when the handset is placed in the cradle (on-hook).

The switches inside the 9K switch pack with their terminal numbers

Note that the individual switches of the 9K switch pack, are of the so-called make-before-break type (MBB). This means that when pressing the button, there is a short moment when all three contacts of the switch are connected together (shorted). This is the opposite of the more common break-before-make (BBM) switch. Although standard switch symbols are used in the diagram above, make-before-break (MBB) switches are sometimes explained with the following symbols:

Three possible symbols for MBB switches.

The individual contacts of each switch are usually named as in the diagram above. The fixed contact, or mother contact, is known as common (c). The contact that is connected to the common when the switch is in rest, is known as the normally closed contact (nc). The contact that is connected to the common when the button is depressed, is known as normally open (no).

The behaviour of the push-buttons can be 'programmed' with a control plate that is fitted behind the spring-loaded latching bracket at the front of the 303 key assembly. It can be installed in four different orientations with two positions each, giving a total of eight combinations as shown here:

Eight possible combinations for the control plate of the latching bracket of the 303 key assembly

For clarity, the notched control plate is shown in red. The presence of a notch allows the button to be latched, whilst (optionally) releasing the other buttons. Note that the middle button (when present) can never be latched. A good suggestion for the scrambler phone is (2). Configurations:

  1. None of the buttons is locked
  2. A is locked and can be released by B or C, C is locked and can be released by A and B
  3. A is locked and can be released by B or C, B and C are not locked
  4. A and B are not locked, C is locked and can be released by A and B
  5. None of the buttons is locked (same as 1)
  6. A is locked and can be released by C only, C is locked and can be released by A only
  7. A is locked and can be released by C only, B and C are not locked
  8. C is locked and can be released by A only, A and B are not locked
No. 303/B key unit
Some telephone sets, in particular the SA5063/1 1 that was used as the voice terminal of the Scrambler Phone from 1944 onwards, were fitted with a No. 330B key unit. Built from the same parts, it is very similar to the No. 303/A Key Unit, but lacks the facilities for the middle button.

Interior front/top view
Interior rear/top view
Close-up of the frame
Top view of the 9K switch pack (303/A unit)
Metal plate for 'programming' the behaviour of the three buttons
303/A key block (9K switch pack)
Pressing the rightmost button
Pressing the leftmost button
Manufacturing codes
Below is a non-exhaustive list of manufacturer codes found on an inside GPO/BT telephones. The list is not claimed to be complete or correct, but might be of use when restoring old telephones. For a more complete overview and additional information, please refer to this website [1].

CB   Central Battery
No dial fitted
F   Figures
Figures (numbers) only on the dial or dial surround
L   Letters
Letters and figures on the dial (or dial surround)
LB   Local battery
Dial fitted
R   Recall
Recall button fitted
Manufacturer IDs
A   Edison Swan Electric Ltd.
AA   Pinching & Walton
AAG   Mullards Ltd
AAR   Associated Automation
AEG   GEC Telecommunication
Glenross (Scotland, UK) [not to be confused with the German AEG]
AEI   Associated Electrical Industries
Later: Henley Cables. Henley later took over Siemens Bros and Thorn EMI and was then taken over itself by GEC Telephones.
AEK   GEC-AEI Telecommunications
Kirkaldy (Scotland, UK)
AK   Peel Conner
This was the telephone manufacturing branch of GEC UK from 1910 to 1921.
Later markings include GEC and AKE.
ATL   Austin Taylor Ltd
Bangor (Whales, UK)
Coventry (UK). Currently known as GPT.
CWL   Pye Telecommunications
Cambridge (UK)
DAE   A.P. Besson Ltd
Hove (Sussex, UK)
DFM   Denis Ferranti Meters Ltd
Bangor (Gwynedd, UK)
E   British Ericsson
Later: Plessey, later: GPT
ET   British Ericsson
Later: Plessey, later: GPT
EET   Plessey
Beeston (Nothinghamshire, UK), formerly Ericsson
EEX   Plessey
Sunderland (UK), formerly Ericsson
EMI   EMI (Valve Division)
Ruislip (Middlesex, UK).
EMT   EMI Telephone Division — unconfirmed
Manufacturer of Privacy Set No. 8.
FB   Post Office factory
Birmingham (UK)
FBR   Post Office factory
Birmingham (UK) Refurbished items
FED   LM Ericsson
FH   GPO Factory
Holloway, London (UK)
FHA   GPO Factory
Halloway, London (Assembly)
FHB   GPO Factory
Holloway, London (Construction)
FHR   GPO Factory
Holloway, London (UK) Refurbished items
FNR   GPO Factory
Edinburg (Scotland, UK) Refurbished items
FRA   Ferranti Ltd.
FW   GPO Factory
Cwmcam (Wales, UK)
FWG   STC Consumer Electronics
Cwmcam (Wales, UK)
FWR   GPO Factory
Cwmcam (Wales, UK) Refurbished items
G   General Electric Company
GEC   GEC Telephones, Coventry (UK)
Currently known as GPT
GEN   GEC-AEI Telecommunications
Newton Aycliffe, Co. Durham
Aidre (UK). Later renamed TMC.
Manufacturer of Privacy Set No. 8.
GTE   Dennis Ferranti
Ferranti imported or licensed GTE phones, as part of a joint venture with GTE USA and GTE Belgium.
H   Automatic Tlephone and Electric (ATE)
Later marked as PL (Plessey)
HAA   Plessey Strowger Works
Liverpool (UK)
I   Ibex Telephones
Harrow (UK)
IBX   Ibex Telephones
Harrow (UK)
KRM   Krone
Cheltenham (UK)
PEH   Pye
Hastings (UK)
PER   Plessey
Romford, Essex (UK)
PL   Plessey
Later became GEC-Plessey Telephones (GPT). Many of the war-time 300-series telephones were manufactuered by Plessey and will therefore have the PL marking.
PLA   Plessey
PLS   Plessey
Swindon (UK)
PLX   Plessey
NewPort, Gwent (UK)
PX   Phoenix
Hendon, London (UK). Later PXA.
PXA   Phoenix
Hendon, London (UK)
RAA   Landys & Gyr
Formerly eronautical & General
S   Siemens Brothers
Later AEI, later: GEC, now: GPT.
SB   S.G. Brown Ltd.
SEA   Siemens-Edison Swan
London N17 (UK). Later part of AEI.
Woolwich, London SE18 (UK). Formerly Siemens Brothers (S).
West Hartlepool, Co. Durham (UK). Formerly Siemens Brothers (S).
SEW   Siemens Edison Swan
Woolwich, London SE15 (see also SEA).
SLW   Plessey
Wigan (UK)
SPK   Plessey
Speeke, Liverpool (UK)
STC   Standard Telephone Company
New Southgate, London (UK)
STM   Standard Telephone Company
Belfast (Northern Ireland)
TCH   Standard Telephone Company
Larne (Northern Ireland)
TE   Telephone Manufacturing Company (TMC)
Formed 1914. Later manufacturing code 'TEA'.  More
TEA   Telephone Manufacturing Company (TMC)
West Dulwich (London SE21, UK). Formed 1914.  More
TES   Telephone Manufacturing Company (TMC)
St. Mary Cray (Kent, UK). Opened 1939.  More
TEW   Telephone Manufacturing Company (TMC)
Winford (Cheshire, UK).  More
TGR   Thorn-Ericsson
Rochester (UK)
TGW   Thorn-Ericsson
Scunthorpe (UK)
TMA   Telephone Manufacturing Company (TMC)
Airdrie (Lanarkshire, UK).  More
TMK   Telephone Manufacturing Company (TMC)
King's Lynn (Norfolk, UK).  More
TMM   Telephone Manufacturing Company (TMC)
Malmesbury (Wiltshire, UK).  More
TPC   Telephone Manufacturing Company (TMC)
Transmission Division. St. Mary Cray (Kent, UK).  More
TSD   Telephone Supplies Ltd.
Aycliff, Co. Durham (UK)
TTH   Telephone Manufacturing Company (TMC)
Canterbury (Kent, UK).  More
TTL   Telephone Manufacturing Company (TMC)
Livingston, Scotland (UK).  More
W   Western Electric
Later: STC.
World War II
During WWII, most 300-series telephones were manufactured by Plessy and by TMC (also Pye-TMC), especially the ones that were used for the war effort. This means that the most likely codes to be found on these devices are PL, TE, TEA, TES, TMA and GNA.

Reproduction parts
High-quality reproduction parts, such as braided handset cord and line cords, that closely match the original colours and production processes, are available from Chris Elliot, Vintage Telephone, in the UK [5]. Some fully restored GPO telephones are available here as well.

 Visit Chris Elliot's website

Expressions and abbreviations used on this page:

BT   British Telecom
Arguably the largest telecom operator of the UK. Previously state-owned and known as the General Post Office (GPO).
CB   Central Battery system
System in which all the energy needed for transmission and signalling is delivered by the exchange. No local batteries or hand generators are used at the telephone end.
 More [3]
CBS   Central Battery Signalling system
Similar to a CB system, except that the mircophone is powered locally by a battery at the telephone end. Power for signalling is provided by the exchange as in a CB system. In the UK there were three types of CBS.
 More [3]
GPO   General Post Office
The state-owned post and telecommunications operator in the UK, before it was renamed BT and privitised. The GPO was als known as British Post Office (BPO) and simply as Post Office (PO). It is currently known as British Telecom (BT).
LB   Local battery system
System in which a local battery is used for providing the current for the speech circuits.
Magneto   Hand-cranked electrical generator that provides electricity for signalling in an (old) telephone system. In some countries known as inductor, crank ringer, or wake-up unit.
PBAX   Private Automatic Branche Exchange PBX
PBX   Private Branche Exchange
Local telephone exchange or switching system, using inside the building of a private organisation, usually connected to the Public Switched Telephone Network (PSTN) via Central Office (CO) lines.
PL   Plessey
Manufacturing code used on the body of the telephone sets and also inside, often stamped on the chassis. The manufacturer's code is also cast inside the bakelite body.
TE   TMC (see below)
Manufacturing code used on the body of the telephone sets and also inside, often stamped on the chassis. The manufacturer's code is also cast inside the bakelite body.
TMC   Telephone Manufacturing Company
The original manufacturer of the Secraphone in St. mary Cray (Kent, UK) [6].
  1. Handset No. 164 circuit diagram
    First released 25 January 1935. Issue C, 5 January 1968. 1

  2. Telephone No. 332 circuit diagram N-432
    First released 3 October 1951, Issue E, 22 February 1968. 1

  3. Telephone No. 394 circuit diagram
    First released 30 September 1937. Issue A, 25 January 1968. 1

  4. Telephone No. 396 circuit diagram
    First issued 16 July 1937. Issue F, 26 January 1968. 1
  1. Documentation obtained from Robert Freshwater [2].

  1. Robert Freshwater, GPO, PO and BT manufacturers codes
    Bob's Telephone File (website). Retrieved September 2018.

  2. Robert Freshwater, TELEPHONE No. 394
    BOBs Telephone File (website). Retrieved June 2014.

  3. Robert Freshwater, T300 Type Telephone Keys
    BOBs Telephone File (website). Retrieved March 2016.

  4. Robert Freshwater, Ericsson Telephone No's N-1015 - N1017
    BOBs Telephone File (website). Retrieved March 2016.

  5. Chris Elliot, Reproduction parts for GPO phones
    Vintage Telephony (website). Visited March 2016.

  6. Robert Freshwater, Converting old telephones to the UK plug and socket system
    Retrieved March 2016.
Further information
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Crypto Museum. Created: Tuesday 28 January 2014. Last changed: Friday, 06 August 2021 - 06:02 CET.
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