CJF;
I do not see an old mark for a 10 gauge bore. The .775" would be from a re-proof. If this gun predates 1887 then the 11 bore could have been right up just short of 10 gauge of course. Even if not a true "Chamberless" it would appear it at least was over-bored, likely for use of all brass shells. It does indeed appear it started life as an over-bored 12 ga & was subsequently enlarged to a 10 with reproof at that time.

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