The gun at proof was marked 12B/14M. This would suggest full and full choke which was likely .040 thou or so. My guess would be that the barrels have been shortened and possibly backbored which could/would still give you 30 thou in both barrels.A little time with a bore micrometer would give you the current bore diameters and whether there is any parallel flat left at the muzzle. If the current choke tapers all the way to the muzzle this might confirm the above. JMHO, actually it's Just My Humble Guess!!!
The proof marks and look of the gun fits the 1879 date.
