The 12 & 13 would have been the actual bore size at proof. A 13 would fall between .710" & .719", while a 12 would go .729" to .740". It is also noted the bbl marked 12 is also marked "Choke" while the one marked 13 isn't. "Just as a THought", perhaps this was done intentionaly, using a smaller bored bbl on the one for a good fit with a round ball for auxillary use & then the choked bbl made with a larger bore having a choke size about the same as the cyl bbl & thus the bbl would not be ruined if the ball load was put in the wrong bbl. Someone just could have ordered it this way.

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