Platon,
You are correct to say the source was SIGINT, as that was the name for the collection of information from all the radio and signals sources, whether quoting directly from intercepted text (as in Enigma decrypts, referred to as ULTRA) or interpreting information and making a deduction (as in direction finding and Traffic Analysis). The Italian section at Bletchley Park was very successful in its efforts against the Italian Naval machine encoded traffic. This was in part because the Italian navy used an Enigma - type machine, but without some of the refinements introduced to the KM Enigmas. (The Italian machine did not have the 'plug-board' wiring options introduced prewar in German equipment). However, much of the BP success against Italian convoys came from two other sources: (a) lower grade Italian traffic, which was often broken, and (b) Luftwaffe Enigma traffic which was regularly read within hours for the bulk of the war. There were other ciphers that helped, such as the Police traffic, but Luftwaffe liaison officers messages being repeated in other ciphers, both Enigma and hand-cipher, were the day to day key to the contents of Italian messages. The Luftwaffe traffic was also the ‘way in’ to several other German systems, including some of those in use by the Kreigsmarine.

The German B-deinst warned the OKW of Italian vulnerability due to the less sophisticated encoding machines in use by her navy, but nothing came of it. Also, there was also at least one example (never verified but circumstantial evidence) of a KM liaison officer on an Italian submarine having his own Enigma to communicate with his superiors: it backfired, as the cipher he was using was already broken by BP and hence used to help keep abreast of the contents of Italian messages.

In light of the varied sources available on a regular basis, it may be that the messages in ADM 223/31 are from source ULTRA, but without something specific to say so, it is not safe to assume so.

Hope this helps,

Bruce