(1) The notches are attached to the tyres not to the body of the wheel and therefore however the clip (Para. 7) is set the turnover position of the wheel occurs with the same letter showing in the window.
(2) The different wheels have the notches in different places.
(3) Wheels VI, VII and VIII have 2 notches instead of 1.
The 4 Wheel Machine.
10. The 4 wheel machine differs "only" (and this is quite enough!) from the 3 wheel machine in having four wheels instead of three in the machine. There are two fourth wheels Beta and Gamma and two "thin reflectors" Bruno and Caesar and any one of the four combinations of these (Bruno Beta, Bruno Gamma, Caesar Beta, Caesar and Gamma) can be used in the machine. The fourth wheel is always placed next the "thin reflector" and this thin reflector, fourth wheel and three normal wheels occupy the same space as the normal reflector and three wheels.
11. The fourth wheel can be set to any one of 26 positions just as can any of the other wheels but it is never turned over by the third wheel. The four wheel machine can thus be set up in 264 = 456,976 different positions for the beginning of a message - whereas the three wheel machine can only be set up in 263 = 17,576 positions.
12. Unlike some machines (e.g. Typex) only letters of the alphabet and not figures can be encoded on the machine. Alphabetical conventions must therefore be adopted for figures, punctuation, spacing etc.
13. When encoding, the wheel turns over before current enters the machine not afterwards; so if we set the machine to position AAA it will encode at position AAB.
B The Daily Key.
14. The daily key, which told the operator how to set up his machine for the day's traffic consisted of the following (abbreviated names in brackets)
(1) Wheel order (W.O.) Three wheels in a definite order, reading from left to right. There were 8 x 7 x 6 = 336 possible W.O's. On the first of the month holders of the 4 wheel machine were also told which of the four reflector and fourth wheel combinations to use; this combination, say