Renato Gamba didn't take this batch of lugers, so it was sold to Interarms instead, nothing fancy about it, really. Just a deal that went wrong. Mauser would market Gamba made revolvers in Germany and Gamba would sell Mauser guns in Italy. It didn't work out really well, that idea.
It is correct that in Italy the 9x19mm para is prohibited, since it is a 'miltary round'. Most 9x19 guns are rechambered to 9x21 for that matter.
ps: If this really was 'one of 100', you would expect the serial number to be something like 'RG001' to 'RG100'. Since this one is number 504, it really is number 504
The interesting part of these guns is also that the original serial numbers were milled out and that new numbers were engraved in place. It points towards the RG series being made of left-over commercial Mauser Parabellum production.