Correct, 12/1 = .740" - .750"; if rebarreled prior to 1954
The flats would carry the Crown over R if re-proved.
Interesting collection of proof marks. Originally proved 35-36 by 'P' on the upside-down date code, with '12' and '13/1' (.719" - .728") bore
Then reproved 1959 and marked .729"
J. Blanch & Sons original London Proof 1887-1896 with bore 12 (.729-.739").
The time bomb damascus
barrels Birmingham re-proved (tiny Crowned R) 1999 (BD) with bore 18.7 mm = .736" which could have been the original bore
Threaded Mode