Note that the 13 mark does not say it had a .710" bore. It says it would accept a .710" diameter plug gauge to a depth of 9" from the breech, but would not accept the next larger gauge to that depth. Prior to 1887 this would have been the 12 @ .729", After 1887 it would have been a 13/1 @ .719". The bore dia could thus have been anywhere betwee the two measurements. With the 13B/14M marks it may well have been proffed prior to the new rules adding the between sises were enacted. It could have been very close to a 12ga bore, but not accept the .729" plug & still be proofed as a 13.
It was many years after this before actual bore diameters began to be marked, rather that the "Range".
