Realize this gun was proofed prior to 1887 & no "Tween" sizes were used prior to that date. 14 gauge = .693" & 13 gauge = .710". When proofed this gun's barrels accepted a .693" diameter plug gage but would not accept a .710" one. Its bores could have been as large as .709" & still carried the 14 mark.
In 1887 the between gauges were introduced as 14/1, 13/1 etc as well as the chamber marking, as of 12 over C in a diamond.
There were many 12ga guns proofed in the Birmingham house with "12" bores.They simply stamped the size according to what the maker sent them.
