- the gun was proofed for 1 1/4 ounces - does this indicate 2 3/4" chambers despite the 12/C mark?
It's very likely, with 1 1/4 oz proof, that your gun came from the maker with 2 3/4" chambers. There are too many exceptions, I think, to state there's no chance it had 2 1/2" chambers--but highly unlikely. The Brits finally solved that problem in 1925 by adding chamber length as a proofmark, thus removing all doubt. As for the 12 over C, that is correct for a gun with chambers shorter than 3". 12 over LC was for guns with chambers 3" or longer.
