What you are looking at are the proof marks for the bore, not the chamber. However at this time (1874 to 1887) there was no chamber mark (a number in a diamond) so without a chamber gauge, you are left with trying out cartridges in the chamber until you find the right one!
You can derive no helpful information on the chamber size from these proof marks.
What is interesting about this mark, which may have passed you by, is that it is a variant on the NOT FOR BALL mark that was present where an appreciable amount of choke was present, as proofed.
However, when the barrel was not choked as we know it nowadays, but 'recess' or 'jug' choked, it could have the same bore size at breech and muzzle hence the '14B 14M' but was still choked.
There was no need to be worried about using a ball so it could be marked 'CHOKE' rather than 'NOT FOR BALL'.
Last edited by Toby Barclay; 07/26/20 02:21 PM.