A few years ago I read the book 'Ultra' about the Enigma machine - Churchills name for it. It was an invention by a Polish scientist early in the middle years after WW1
I recall that the machine came into - IIRC correctly - French hands very early on it was passed to us and we had it LONG before the machine was taken from the U-Boat.
In any case there is an anomaly here and O B Jones book was very detailed.
If I have got it wrong I will, no doubt, be told but O B Jones was an enigmatic man himself and his book was brilliant - the whole story of Bletchley Park and the maths genius Alun Turing.
Having said I have looked up a few relevant details and this link may help prove the point about the U-Boat.
Deception - Ultra and Enigma