That drawing came from 'damascus knowledge' I think; it might for damascus barrels. I measured three 12s that I have at the various points and none have the OD that the barrel on that chart has.
That is one reason why I hesitate to post it at times. I have measured many more guns and never found one with breech walls that thick. BUT, measuring them has saved me grief and perhaps a finger. It is amazing at how many have been poked at. I have a beautiful German gun with Bernard damascus. Some nimrod got in there and honed the heck out of the forcing cone. I do not consider it a safe shooter.
I think the point is to measure and look for sudden changes. Use common sense and the proper ammunition. Many of these guns can last another 100 years if properly cared.
Pete