A (very likely stupid) question: do those 75mm chambers mean that, all other things being safe, this gun should be OK with standard US 2 3/4" lead shot ammo?
I would stay away from standard US 16ga ammo. The gun is OK in terms of chamber length, so there's no issue with additional pressure due to a longer shell in a shorter chamber. However, the pressures developed by US factory ammo are a good bit higher than the loads for which this gun was built. There's also the issue of heavy loads such as the 1 1/4 oz 16ga "magnum", which would almost certainly be bad for the wood on a gun of this vintage.
If one is into reloading, standard American hulls loaded to lower pressures should be fine. Factory shells from American specialty outfits like RST and Polywad would work well, as would European factory shells.
