The Uhr Box is an attachment added to the service Enigma in 1944 by the Luftwaffe. It was connected to the plugboard with 13 pairs of double ended plugs.A forty position (00-39) switch selected one of forty different Stecker wirings, for each plug arrangement. The internal cross wiring of the Uhr is arranged for every fourth position of the switch to produce a reciprocal substitution for 20 letters, the other six being 'self-steckered'. All other positions are non-reciprocal substitutions for twenty letters. At position 00 the Stecker connections are identical to the ten plug connections.

For a full description of the specification of the Enigma Uhr, see the paper:
Ulbricht, Heinz. 1999. Enigma Uhr Cryptologia 23(3) 193-205.

For some background on the usage of Uhr, see the paper:
Marks, Philip C.2001. Umkerhwalze D: Enigma's rewirable reflector - Part I Cryptologia 25(1) 101-141.

Enigma Uhr

Click Here to download The graphical
simulator for this cipher machine.
(Requires Windows 3.1/95/98/NT4/2000/XP
with at least 800x600 display
and 16 bit color)