Note that most of the muzzle loaders were built prior to 1887, so they do not have the tween sizes, only whole number sizes. My I Hollis M/L double with Birmingham proofs is marked 12. On M/l'ers I believe the normal practise was to gauge it at the muzzle.
My I Hollis has the mark which indicates it did not undergo provisional proof, only final/definitive proof but with the provisional charge. I forget the date this was introduced but would make it a quite late for a M/l'er.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra