Here is a picture of the proof and other marks on the underside of the barrel:



I believe that the barrels, used in a muzzleloader conversion to an underlever breach loader, were 13 gauge rechambered to 12 gauge for the conversion. I measure the bores at .702 and .707, on the cusp of 13 and 14 gauge according to Drew's chart. I note that there is one provisional Birmingham proof mark and two final black powder proof marks. Does this mean that it was tested twice?

Many thanks for everyone's input.

Rich