... not by latitude and longitude.
Thus position AB1234 would indicate point, say 55 degrees 30 minutes North, 25 degrees 40 minutes West.
This of course presented no problem to us once a portion of a gridded chart had been captured and the whole reconstructed.
But in (September) 1941 the Germans started transposing these letters, square AB becoming, for example, XY
while a figure would have to be added to or subtracted from the numerals, so that 1234 would appear in the text of the signal as, say, 2345.
These transpositions were changed at regular intervals.
Yet either the Enigma was being read, in which case these precautions were a feeble response, or it was not, in which case they were a waste of time.
The rationale lay not in foiling the British cryptanalysts, but in creating a defence against imagined spying and treachery.
The cumbersome disguise succeeded in confusing their own officers:
On one occasion we successfully solved a disguised grid reference and diverted a convoy clear of a waiting patrol line,
only to find that the C.O. of one of the U-boats involved had not been as clever as we had and had misinterpreted the disguised grid reference given in his orders and blundered into the convoy in consequence.
In November 1941 the system was made even more complicated, and gave rise to long periods of uncertainty at Bletchley.
They were still on a knife-edge, and were never allowed to forget it.