The gun may have been made in 1879 but the barrels are marked "not for ball" and this marking was discontinued in favor of "choke" in 1887. It's possible that the barrels were made and proofed several years prior to the final assembly of the gun.

The 12B/13M markings indicate a gun which was 12 bore choked to 13 bore (12B "bore", 13M "muzzle").