It seems like bores are large on early Parker Bros, guns from the black powder years, when the practice of using brass shells with one gauge larger wads was common. In the 1893 or 1894 time frame, when factory loaded paper smokeless powder shells were coming on strong, the Brothers P reduced the bore size and began advertising their guns as "bored for nitro powders". My 1889 12-gauge GH-Grade has bore diameters of .757" seriously overbored, while my 1894 vintage 10-gauge NH-Grade has bore diameters of .779" pretty close to gauge.
The above is very simplified, and things get more complicated when one gets into the 12A & 12B and 10A & 10B shells. Likewise, up to about 1931 all our North American shotshells used corrosive priming, so bores got pitted and polished out on a pretty regular basis, leaving it very hard to state unequivocally how they left the factory. Then throw in that Stan Baker didn't "invent" overboring, it was practiced by various gunsmiths and factories through much of the 20th Century. On and on.....
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