CJF;
12/1 = .740", there was no 12/2. If my memory is correct the 2 steps "Tween" gauges began at 10ga, I know it was larger than 12. From 11 gauge down there was only the /1 tween size. It would appear it was originally proofed as an 11 gauge, which would have put the bore between .751" (11) & .763" (11/1) assuming post 1887 manufacture.

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