nominal 16 gauge = .662" (16.8mm); 15 gauge = .677" (17.2mm).
16/1 = (.662" + .677")/2 = .6695". Note average of the two numbers in calculating the "Tween" size is dropped, not rounded, to 3 decimal places, thus the 16/1 is .669" rather than .670".
!6/1 marked on the barrel meant that at proof it would accept the .669" plug gage to requisite depth but not the .677" one. There was no minus tolerance on the bore mark as the barrel had to accept the gage of that diameter. Plus tolerance was to the next size gage in the series which varied with each gauge size.
Gages were made in every even size gauge from 1 down to 50 (.453") & then in .010" steps from .450" down to .300 caliber (172.28 gauge). The .410 is incidentally a 67.49 gauge.
