anyways, the few prototypes didn't really have a defined case number. Usually these protos were attempts to improve the 3646 case which suffered from the fragility of the wire lugs. The thicker lugs were welded and then the case sand-blasted to hide the welding,,,,a similar case type exists in the Panerai museum collection and was displayed at the SIHH a few years ago when the Minerva models were introduced.
Have a great holiday season, all the best to your lovely family!
Cheers and best from the holyland,