It is a pre 1887 proof & at that time chamber was not marked, only the bore. Also at this pint the "Tween" gauges were not in use thus a bore marked 12 could be anywhere from just accepting a 12 gauge plug gage (.729") up to just not accepting an 11 gauge (.751") one. It is truly hard to ascertain what the original chambering was. Several Birmingham proofed guns have been mentioned here previously, including one of mine, which are marked 14 but have 12 gauge chambers.
Agree with Ted, that looks like a concave rib, but not a swamped one.
