I would not call this box rare, but no one is tripping over them on the sidewalks, either. Rare, scarce or common, it is a nice box and a good add to any collection of military ammo boxes and/or auto pistol cartridge collections.
The “C.A.” is the Chief Inspector’s initials representing Cavallo Alfredo (Alfredo Cavallo to an American) who was at Pirotecnico di Bologna from about 1939 to 1942. The “B” on the headstamp and the “P.B.” on the label stand for Pirotecnico di Bologna, the manufacturer.
When I got my first box like this a jillion years ago, I was as much a pistol collector as a cartridge collector, and because of the caliber, I thought the “P.B.” stood for “Pietro Beretta,” as in the maker of the Modelo 1934 9 mm Corto Pistola. I’m better informed now - no smarter, just better informed.