Gentlemen, It has always been my understanding that the dividing of bore sizes into smaller portions by the use of fractions, occurred with the Proof Rules of 1887. ie 12 which previously
spanned .729-.751 was divided into 12 as .729 - .740 and 12/1 which covers .740 - .751 . I have not been able to find a confirmation of this in print via the internet. Do any of our knowledgeable throng have a definite yay or nay on this???
Thanks

Better to keep your mouth shut and be thought stupid,than open it and confirm.