Saku, WE need to know what number is in the diamond shape proof mark.That is the chamber size.It will be either a 12 over C, or 10 over C . By the Proof Rules up to 1887,10 ga. guns that have slightly undersize bores at proof,ie. under .775, would be stamped 11 .I have seen plenty of these.On the other hand,an overbored 12 made for thin brass cases,which measured over .750 at proof would also be stamped 11. Check out those diamonds, or try and chamber a 10 ga. cartridge.

Better to keep your mouth shut and be thought stupid,than open it and confirm.