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Enigma D
Commercial Enigma A26

The Enigma D was developed in 1926 as the successor to the Enigma C. The official model number was A26 and it was given the internal designator Ch. 8 by the manufacturer Chiffriermaschinen AG. It was replaced a year later by the nearly identical Enigma K (A27).

The Enigma D contained many improvements over the Enigma C. First of all, the top lid of the machine was made more accessible, so that it was easier to alter the basic settings (key). The three coding wheels were now mounted on a removable spindle, so that the order of the wheels could be changed as well. Furthermore, the reflector (UKW) became settable, which means that it could be set to any of 26 positions. All this increased the maximum number of permutations.
 
As a result, 4 wheels protrude the top lid and hence there are 4 windows through which the current settings can be viewed. Because of this, it is sometimes assumed that this is a 4-wheel Enigma machine. Although stricktly speaking there are 4 wheels, the leftmost one is the UKW. It should therefore be defined as a 3-wheel machine with a settable UKW.

The machine was built on an improved die-cast chassis and the order of the keys and the lamps was now similar to that of a standard German typewriter (QWERTZ... rather than ABC...).
  
Typical view of Commercial Enigma with serial number A818

Looking at the Enigma Family Tree it is obvious that around 1926 the Enigma D was the core product of the manufacturer, Chiffriermaschinen AG. All further Enigma machines would be based (largely or in part) on the Enigma D design. The Enigma D itself was short lived and was replaced approximately a year after its introduction by the nearly identical Enigma K.
 
The wooden case, placed straight up, ready for shipment. The machine has a leather carrying strap at the back (don't use it now, as it is likely to break) The lock that keeps the wooden cased closed. This lock would be used on all further Enigma machines. Wooden case of the Commercial Enigma Commercial Enigma with open case Just below the name tag of the manufacturer (Chiffriermaschinen AG) is a badge that says 'Made in Germany'. The top lid holds the spare light bulbs, the Enigma badge, the sunlight filter and the name of the manufacturer. The A818 with open lid, seen from the front.
Typical view of Commercial Enigma with serial number A818 The bare Commercial Enigma machine taken from its wooden carrying case. A lamp lights up when a key is pressed Note the shiny details on the body of the machine. The leftmost wheel is the settable UKW. Reflector (Umkehrwalze, UKW) The wheels after opening the top lid A small shiny clip is used to hold the green sunlight filter in place over the lamp panel. Turning the machine on

The images above were taken from the Commercial Enigma machine with serial number A818 that surfaced in 2011. The machine was found in near mint condition and still had its original (commercial wiring) [1]. Research has shown that this machine was produced or sold in 1927 [2]. It is therefore difficult to say whether it is an Enigma D (A26) or an Enigma K (A27). Apart from some minor production differences, these machines are identical.
 
Wheels
The Enigma D was a machine with 3 coding wheels and a settable reflector (UKW). All 4 wheels protrude the top lid of the machine. As the UKW has a thumbwheel for setting its position, it is nearly identical to an ordinary coding wheel. As a result, the machine is sometime erroneously thought the be a 4-wheel Enigma. The UKW does not move during encipherment.
 
The wheels are made from steel and aluminium, and have a Bakelite inner core that holds the 26 contacts at either side. Each wheel can be set to one of 26 positions, each of which is identified with a letter of the (Latin) alphabet (A-Z).

The image on the right shows a close-up of wheel number III. As you can see, the aluminium thumbwheel is nicely polished and has rounded edges. This was common for commercial Enigma machines in those days. Later machines, such as the military Enigma I were less shiny and were less well finished.
  
Close-up of the index disc of an early Commercial Enigma wheel. Note the polished edges.

More details of the wheels (including the ETW and the UKW) in the images below.
 
Four wheels protrude the top lid. The leftmost one is the reflector (UKW). The other 3 are the coding wheels. The four wheels after opening the top lid The three wheels of the A-818 (with commercial wiring) Close-up of wheel III of the A818 Reverse side of a wheel Polished thumb wheel Reflector (Umkehrwalze, UKW) Entry wheel (Eintrittswalze, ETW)
Close-up of the index disc of an early Commercial Enigma wheel. Note the polished edges. The letter ring on a wheel of the A818

 
Wheel Wiring
The wheels of Enigma A818 (see above) contain the standard commercial wiring as presented in the table below [1]. This wiring was identical for all commercial machines, including the later Enigma K (A27). Although the wiring of the wheels was changed by some customers, they often left the wiring of the UKW intact. As far as we know, the wiring of the ETW was never changed.
 
Wheel ABCDEFGHIJKLMNOPQRSTUVWXYZ Notch Turnover #
ETW QWERTZUIOASDFGHJKPYXCVBNML      
I LPGSZMHAEOQKVXRFYBUTNICJDW G Y 1
II SLVGBTFXJQOHEWIRZYAMKPCNDU M E 1
III CJGDPSHKTURAWZXFMYNQOBVLIE V N 1
UKW IMETCGFRAYSQBZXWLHKDVUPOJN      

 
Instruction booklet
Operating instruction for the Enigma are very difficult to find. This is partly caused by the fact that the machine is extremely easy to operate, whilst most users (e.g. the German Army) developed their own operating procedures. In many cases the instructions where no more that a few typed A4 pages, such as with the Enigma A28 Zählwerksmaschine (see: download).
 
For the common commercial machines, however, a small 20-page booklet with a clear description and simple operating instructions was released. It was also used as a brochure.

The image on the right shows such a booklet. The first page shows a picture of a commercial Enigma machine, plus the name and address of the manufacturer Chiffriermaschinen AG in Berlin. The book in the images is aged somewhat by moist after all these years, but is otherwise in excellent condition. Instruction booklets like this one are an extremely rare find.
  

The front cover of the booklet shows the serial number of the machine that it was issued with, written with a red pencil, in the top right. In this case, it exhibits serial number A833, which is in the same range as the Enigma machine (A818) shown above.
 
Cover of the Enigma instruction booklet The somewhat embossed title Page one of the booklet Page one of the instruction booklet Close-up of the Enigma machine shown on the first page

 
Descendants of the Enigma D
In 1927, one year after the introduction of the Enigma D, several developments of improved machines were started. This lead to a range of new - improved - commercial machines and advanced high-end machines. The later military machines were also initially based on the Enigma D. The following machines are directly developed from, or inspired by, the Enigma D:
 
  • Reichswehr D (Ch. 11a)
    Machine with a single-ended Steckerbrett. This ultimately lead to the developement of the later Enigma I (Ch.11f) used by the Reichswehr (later: Wehrmacht).

  • Enigma K (A27, Ch. 11b)
    From 1927 onwards (right up to 1944), this was the main non-Stecker machine. Many improvements were made and many different versions exist. It was, for example, the basis for the Enigma T (Tirpitz), the Swiss K variant and the Enigma KD.

  • Zählwerk Enigma (A28, Ch. 15)
    This was a range of high-end Enigma machines with advanced mechanics and enhanced cipher security, such as multiple wheel-turnovers. The later Enigma G range (G31) was also based on this machine.

  • Enigma Z (Z30, Ch. 16)
    This was a numbers-only version of the Enigma machine. It had just 10 keys (0-9), 10 lamps and the wheels each had 10 contact points.

References
  1. Paul Reuvers and Marc Simons, Wiring of Enigma A818
    Wiring of the A818 verified in October 2011 as commercial wiring.

  2. Frode Weierud, Enigma serial number research
    Forthcoming publication.

  3. Chiffriermaschinen AG, Enigma booklet
    Instruction booklet with images.

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