I think that in the early years they were all stamped on the barrels rather than the flats. This gun is pre chamber mark which as I recall was 1887. The tween sizes for the bore mark were introduced at the same time so with the 13 mark the bore could have been anywhere from accepting the 13ga plug (.710") to just not accepting the 12ga one (.729").

Miller/TN
I Didn't Say Everything I Said, Yogi Berra