Hi David, I think you will have to research this as I may be wrong, but I understood that the size stamped was the largest plug gauge that would go 9" into the bore - so if 0.719 goes, but 0.729 doesn't, it is stamped 13/1.
13/1 was quite common.
0.727 and 0.728 are what would be described as 'marginal' if I am right and the 0.729 plug passing would mean 'out of proof'.
Your wall thicknesses seem to leave plenty of scope for any future work, but in the UK, reproof would be required if the next proof plug would now fit.
This is a legal requirement if the gun is sold, but is not a legal requirement for use (though an insurance claim on an out of proof gun might be interesting!
Please do check up on this because I have not read up on this in a while!
John
