Hi Pooch, as been said, the 12C, just means that the gun is a 12b(ga) gun. It has nothing to do with chamber length. It does not matter what proof house, London or Birmingham.
Army & Navy did not make any guns at all, they were an oulet, that when they first started supplied, various sporting goods (to include guns of course) to Officers of the Army and Navy of the UK. Most of the guns were supplied by the Birmingham trade, but a gun could be proofed in either house, regardless of the region, maker or where the gun was sold. Charles Boswell was a London Maker who had his guns proofed in Birmingham after a "disagreement" with the London Proof House.
If you can, please post pics of the bbl flats, that sure would help!
Col. I stand correct on my one post, I done forgot about the Rules of 1904-1925 and shot charge....good catch.
Best of Luck!
Greg
