As yankee wrote, it‘s one of the Italian CA/CB boats, and quite probably, it‘s the CA 2 after its modification, including removal of the torpedo tubes, and replacing them with 8 explosive charges of 100 kg each, removal of the conning tower and of the Diesel engine, turning it into a purely electrical driven sub for attacking allied ports. As a mother ship, the 1,200 ts sub Leonardo da Vinci was modified, but never used in this role for real attacks.