.751" is 11 gauge, 12/1 which began to be marked in 1887 in British proof is .740". In British proof when they were still marking by gauge rather than actual diameter a 12 marked barrel could be anything which would accept the .729" plug gage but would not accept a .740" one. From .740" up to .751" it would be marked 12/1. 13 gauge = .710" so 13/1 = .719. Any of these sizes could "& Were" chambered for 12 gauge shells. I even have one pre 1887 gun chambered for 12ga with bore marked as 14 (.693"). Prior to 1887 there were no "Tween" gauges so 14 could have been anywhere from accepting the .693" plug up to just not accepting a .710" one. In so far as I am aware of there were no legal requirements as to what a bore diameter had to be in relation to it's chambering. The gun was proofed according to it's actual bore diameter. My understanding was that when the bores were marked by Gauge, the gun was considered out of proof when it would accept the next larger gage than it was proofed to. fter the actual bore diameters began to be marked the .008" went into effect. Thus under the old system a gun having a .728" bore would have been proofed as a 13/1. As soon as it would accept a .729" gage it would have been Out of Proof. Under the new system it could go up to .736" & remain in proof.
The English gauge system listed all whole numbers from 1 through 50, 50 gauge being .453". Below this gauges were calculated for diameters in .010" steps from .450 down to .300" with the gauge carried to two decimal places. .450" =50.04 gauge with .300" = 172.28 gauge. A .410" bore incidentally is 67.49 gauge.
