Just a bit outside my wheelhouse, but, I’ll give it a shot...
The gun was originally proofed at 13 bore, or, 728. This was common, I’ve owned several 12 gauge English guns that were proofed the same way. The marks for 3 tons, 2 1/2” and 728 are all key to determining if the gun remains in proof. If the bore hasn’t been honed more than .010, or, the chamber lengthened, you should be good to go with CIP 2 1/2” ammunition. There is (was?) choke in the left tube when it was proofed, in that era I don’t believe they indicated how much choke, just that it was there.
Not sure what other questions you may have.