The latest issue of the Parker Pages on p. 30 addresses these issues and states that almost all 12ga guns less than SN 70,000 have bore diameters of 0.750- 0.760.
My guess is that the barrels were reamed to "clean up" and the reaming stopped short of the choke area, leaving some choke. The wall thickness on these black powder guns is substantial, significantly thicker than the tapered, thinner barrels for the smokeless powder era.
Bruce Day
Parker amateur beginner
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