Realize that prior to 1887 there were no "in-between gauges. A bore proofed as a 12 could measure anywhere from about .730" up to .750".
I have an old Birmingham proofed W Richards, built by Cabrough which is chambered for 12 gauges shells with the bore proofed as a 14. I have seen pictures of a couple more of these identical guns which were also proofed as 14 but chambered for 12.

I agree with JBL. it was likely built this way as a tight-bored 10.

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