Gunmakers and gunsmiths managed to bore chokes for about 100 years before there was such a thing as a Skeets bore gauge. LC Smith used plug gauges, and the drawings for those gauges are illustrated in Brophy's "Plans and Specifications" book. Any machinist worth his salt can use a set of inside calipers and his micrometer to accurately measure both the bore and the choke at least as well as with a gauge of the type that Skeets is.