I would have to look up to see what marks applied to breech loaders, if any, which did not apply to muzzleloaders. Also a single barrel may be different from a double. It is noted however the vast majority of muzzleloaders were cylinder bored. For the muzzleloaders it was I believe normal to measure the bore at the muzzle end. The "Not For Ball" mark was not applied to a cylinder bored gun. With the 13 mark the bore (At the muzzle) could have measured anywhere from just accepting a .710" plug gage to just not accepting a . 729" one. Prior to 1887, as I recall the date, there was no 13/1 mark. If you plan on shooting it make certain to use wads which give a good snug fit, you may well be able to use regular 12ga wads.
My I Hollis double muzzleloader is marked 12 but I have to use 11 ga wads for a good fit.
