Binko;
The left barrel was stamped by the proof house as a 15ga (.677) & the right one as a 14ga (.693). 13ga would be .710" & 12ga .729". This gun was proofed prior to 1887 when the chamber mark was added & at this point no intermediate sizes were marked. Each mark could mean the diameter was somewhere from the designated diameter for that gauge up to just under the next larger gauge. The actual size was not measured but plug gauges of the sizes were simply tried in the bore & the barrel stamped according to the largest that entered to the specified depth.
Another thought comes to mind. Throughout the muzzle loading days & on into early breech loaders before choke boring became rather common bores were gaged from the muzzle. This gun may well have been an early choke bored gun, but was still gaged from the muzzle.
