Bruce,

The reference numbers in ADM223/31 for the HMS TURBULENT's episode may be ZTPI numbers but the heading of the column is blank (so I cannot be positively sure, the same column has a ZTPI heading in most pages of the volume).

As far as I know ULTRA's signals were not solely derived from Enigma decrypts but from other sources such as the Italian C 38m machines (of Swedish origin) which was used for Italian shipping and this was I think the main source for this O.I.C. volume. I think the Italian Navy was not using Enigma machines at this time, it was abandoned in the summer of 1941.

I agree that the GAF traffic was probably the most reliable source of information. The Italian Naval grid system was much more secure than the German one, and as far as I know it was never broken, except perhaps very briefly through captures.

I am curious about the KM liaison officer on an Italian submarine, I have not yet found the evidence of one. What is your source? I did find evidence that Italian naval personnel at Betasom were instructed to the use of the German Enigma coding machines but this was probably to keep in touch with the Seekriegsleitung. Italian submarines did not communicate directly with German U-boats except by visual signals when in the vicinity, communications went through their respective headquarters.

Best regards,

Platon