I have no objection to anyone posting my at-a-glance guide to dating guns by proof mark. However, Nigel Brown does a more comprehensive job in the back of British Gunmakers - he had more space!

Basically, as long as the bore is less than 10 thou bigger than it was stamped at the time it was proved, (and is not materially weakened by deep pits, or altered by lengthened chambers, screw-in chokes etc) it is in proof.