The 2-3/4" chamber is a plus, that may indicate it was meant for the North American market. The 12 C in the diamond is the nominal gauge callout and the 13/1 is the bore size limit (indicates a 0.719" bore diameter). I cant see it clearly enough but are the other two marks between the 13/1 and the 12 C a crown with two crossed scepters? One with a V below the crown (the view mark) and the other with a B-P-C at left-center-right (the proof mark)? I think the other two in the bottom picture are provisional proofs but can't really tell. Is there a "Nitro Proof 1 1/8" or similar marking not shown in the pictures? Otherwise, the marks look consistent with Birmingham proofs of the right time period (up to 1904).