Raimey
All that you say here is certainly to the best of my knowledge correct in it's entirety. I am not positive as to the earliest date for the standardizing of the British Gauge dimensions. Even this of course did not mean that every bbl stamped 14 for instance left the factory at an exact .693" size. I have for instance an old side lever opener, backaction hammer, W Richards bearing Birmingham proofs. This gun predates the introduction (1887) of the in-between stamps (14/1 etc) & has a 12ga chamber (no stamping) & bore marked 14. It is cyl bored both bbls, so no B/M stamps. It is quite possible indeed it had bores very near to the .710" (13ga) which it now measures. That 14 stamp simply meant it would accept a .693" plug, but not a .710" one. That's .017" leeway. To say because thiese bbls are stamped 14 they went through proof at exactly .693" would simply be wrong, but to state that 14ga is a "nominal" .693" size is correct. Charts from the British proof houses dating back to at least 1855 give the same sizes still in use to present time, however a chart given by W Greener in his book "The Gun 1834" stated to be taken from "a small work, published by the authority of the Proof Co" show all sizes larger than later ones. It would seem that at this time the ball was calculated to be of exact weight for the gauge & likely the bore was designed to give clearence.
A 14ga gun under this chart was proofed with a 1.142oz (16/14) ball, .875oz (14 drams) of powder & had a listed bore dia of .701". This was .008" over the later listed .693" of a 14ga. Many times on these very early guns about all we can do is speculate as to what the makers intent was.