Code and ciphers: Julius Caesar, the Enigma and the internet

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chapter 9
Figure 9. 3.
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during the encipherment of the six-letter double indicator there is an 80%
chance that only wheel R1 moves.
For wheel R2 can only move if R1 is on a notch position and on the original
Enigma there was only one notch on the 26-position notch ring. Thus
if the notch was effective when R1 was in position Z it could only cause R2
to move during the six-letter encipherment if the original setting of R1
was U, V, W, X or Y. (If the original setting of R1 was Z wheel R2 would turn
immediately before the encipherment of the first letter and would remain
there for the next five letter encipherments unless R2 itself was now at its
notch position and so caused both R3 and itself to move at the next letter
encipherment; this hardly affects the probabilities and, for simplicity,
will be ignored. The cryptanalytic attack is easily modified to cover this
unlikely possibility.) Therefore there are, normally, 21 of the starting
positions of R1 which ensure that R2, and therefore R3, remain stationary
throughout the encipherment of the six-letter indicator and so the probability
that only R1 moves is 21/26, which is more than 0.8. Had there
been more notches on the notch ring the probability would obviously
have been lower. In the later stages of the War some notch rings had two
notches and the notch rings of the Abwehr Enigma had many more, as we
see later.
If R2 and R3 don’t move during the encipherment of the six-letter
indicator the Enigma becomes, effectively, a one-wheel machine, consisting
of R1 and a composite reflector, made up of U, R3 and R2, none of
which move during the encipherment. This is illustrated in Figure 9.3.
The composite reflector, formed by the stationary wheels R2 and R3
with the fixed reflector, U, is on the left. R1, the only wheel that moves
during the six-letter encipherment, is on the right. The internal lines
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