The 732 is either the maker's serial number or a batch number to make sure the parts for each gun stayed together during manufacturing.
The French gunmakers marked the bore diameter in millimeters: roughly 18.1 through 18.3 for 12 ga, 16.9 through 17.1 for 16ga.
I have a similar gun in 16 ga, sans the carved breech balls, plus some remaining case color. It, too, was a war trophy - the seller's late GI father found it in a Norman barn in the summer of '44, liberated it and sent it home, where his dad used it for a few years and then it sat unused and in a place of honor in the vet's living room for over 50 years.
These are great guns, made more to carry a lot and shoot a little. If, as with yours (and mine) the barrels are solid, they're as good as anything out there.
