The gun in question though is said to have 1887-1896 proof marks. The 1954 rules would not apply except in case of a re-proof & no mention was made of this. The plain 12 without the diamond or C as stated indicates bore diameter, but not necessarily exactly 0.729". What it meant was it would accept a 0.729" plug gage to the required depth of 9" from the breech but would not accept the 12/1 gage of 0.740" diameter, could have been anywhere between those sizes.

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