Trying to help Tom, but I'm always wrong re: British proof marks frown

The 'B' (breech) and 'M' (muzzle) marks were used 1875-1887, as was "NOT FOR BALL".
I do not know how far from the breech and muzzle these numbers were measured; post-1887 it was at 9".
11 = .751"
12 = .729"
14 = .693"
16 = .662"