Note that 13 gauge as marked for the original bores is .710" in size. How ever as Steve says this gun was shipped in 1872 it would precede the use of a 13/1 mark. Original bores then could have been anywhere from accepting a .710" diameter plug gage to a depth of 9" from the breech up to just not accepting a 12ga (.729") plug. Technically as it would still not accept the .729" plug it would still be "In Proof" from a standpoint of the bores.

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