Larry;
One thing here I am not following you on. For instance I have an older Birmingham proofed 12ga with both bbls cylinder (pre 1887). The bbls are proofed as 13. That's all as far as size. There is no B & M marks & no Not for Ball. Does it not follow the exact same rules you stated, ie in proof from .710"-.728". Seems to me the proof limit applies only to the bore dimension with no reference to the choke.
All keep in mind though & I don't recall the exact date of switch, but the 9" from breech measurement came into being with the breech loaders. Throughout most of the muzzle loader reign, the bore size was measured "At the Muzzle".
