As best as I recall the inbetween sizes were started with the proof rules of 1887, so yes prior to that a 13 could have been from .710" to .729" A 13/1 would of course have never been other than .719"-.729"
Gauge diameters are calculated to the nearest .001". The in-between sizes depending on whether there are 1 or 2 steps are either half way or 1/3-2/3 of the difference between gauges. On these any numbers beyond 3 decimal places are simply dropped even if .0005" or above. Thus .710" + .729"/2 = 1.439"/2 = .7195". The .0005 is dropped so the 13/1 is listed in the proof charts as .719". In this case if you calculated 13/1 as a 12˝ gauge it would figure to .7193" but this is not the case for all gauges & especially with the gauges having two steps.
