Miller, all of his 4 bores that I measured were either 12" from breech or 4" from muzzle and ranged from .928 to .938. An H. Holland single barrel had a bore of .941.
I do believe in what you are saying in that each maker made them to their size.
The rim diameter of the 4 bore shells I measured were 1.180 and the base 1.065. These shells were 4 1/4" long.

Buck was looking for his ultimate turkey gun.

David