Hi Siri,
yes, according to the report on the loss of HMAS Sydney, the CL asked Kormoran (disguised as a Dutch m/v) for her "secret wartime call letters" rather than the ones she had originally flashed. I am not sure this applied to all vessels but it could be an avenue to explore. I have long forgotten my Morse code, perhaps someone better equipped can detect whether there is a similarity in the transmission which could have fooled the operator aboard Marcello. When you are broadcasting letters without a finite sense to a word, it is possible there was a misinterpretation of their sequence.
Regards, Brian

Tuez les tous, Dieu reconnaitra les siens,
Arnauld Amalric, Béziers, 1209 AD