The proofs definitely appear to be Birmingham. My memory may not be right but I was thinking the chamber mark, IE, 12/C was not used until 1887. At the same time the "Tween" bore sizes were added as in 13/1 so while this one was proofed as a 13 it could actually have been a bit larger, just not enough to accept a 12.gauge plug gage.


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