A few more words on the "Proof". Larry quite accurately answered the enclosed 12/C mark. The 12 mark indicated bore dia at time of proof. This was marked by determining the largest of a series of plug gages which would enter the bore for at least 9" from breech. 13 =.710", 13/1 =.719, 12 =.729, 12/1 =.740 & 11 =.751. It is not really that uncommon to find a gun with 12ga chamber having any of these bore marks & even on occasion a wider variation.
I also am operating from memory here Larry, but my recollections are when the "Choke" mark replaced "Not for Ball" it was made mandatory on any bbl having a choke constriction of as much as .008" (0.2mm). This change was done to counteract the belief held by many that even a ball made suitably undersize to pass through the choke was still unsuitable for use.
In black powder days the standard load for the 2½" 12ga gun had been established as 3dram-1 1/8oz & for the 2 3/4" gun 3¼dram-1¼oz. As smokeless powders took over from black, loads having similar ballistics also became standard & this wt of shot charge became a part of the marking at proof. It took until 1954 to alter these marks from one of wt to one of pressure. However for many years prior to this shells had been specifically tailored for use in nominal 2½" chambered guns, & which did not violate the 1 1/8oz proof, which contained shot wts of at least from 15/16oz up to 1¼oz. This was done primarily by altering velocity & eventually by adopting new powders to maintain normal pressures. It must of course be noted the 1¼oz Standard as loaded to normal velocity at a higher pressure level for 2 3/4" chambered guns "Did Indeed" violate the 1 1/8oz proof. This of course led to a good deal of confusion, much of which still lingers around to the present day.
