binko,
The proof marks on the barrel of your gun indicate that the original calibre of the gun was 14G. Many of these vintage 14 bore shot guns,[for which at one time shells were available], were re;bored to 12G by unskilled gun smiths. Hence the odd- ball chamber size you have encountered. In my opinion 10/11/12G proof or chamber size was never applicable to your gun at the time it was proof tested.

Last edited by Roy Hebbes; 03/30/14 10:58 AM.
Roy Hebbes