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When the Germans added wheels or other features, did they reuse the old wheels, so 90% of the machine was identical to the previous version?

six plugboard leads, leaving 14 letters unsteckered. January 1939 when the number of leads was increased, leaving only a small number of letters unsteckered.three different rotors for the three positions in the scrambler. (This continued until December 1938, when it was increased to five and then eight for naval traffic in 1940.)By delivering 4 extra leads, or 1 extra wheel, the number of permutations increases dramaticallyThe 3-rotor scrambler could be set in 26 × 26 × 26 = 17,576 ways, and the 4-rotor scrambler in 26 × 17,576 = 456,976 ways.With six leads on the plugboard, the number of ways that pairs of letters could be interchanged was 100,391,791,500 (100 billion) and with ten leads, it was 150,738,274,937,250 (150,738 billion).

Adding a few new wheels makes much more than a 10% change to the behaviour of the machine.In delivering 2 extra wheels, the internal wiring of these new wheels was initially unknown, and each wheel could be set in 26 initial positions. So this prevented decryption of the traffic on most days, until the circuitry of the new wheels was deduced.