Jeff,
You know these guns well, and recognize just what this gun is. I'll also wager that the bore is .322 or .323, as in "S" bore. The little pin plunger in the extractor is a hint.
Some pretty cagey makers of the better quality guns made them to shoot all flavors of the 8MM round adequately. They used the .321/.322 bore barrels so that one could shoot J bore or S bore ammo. The J bore wasn't quite as accurate, but it shot well enough for the relatively close shots from low or high seats, while the .321/.322 barrels were exceedingly accurate using .323 ammo and had no pressure issues. That little pin plunger made it so one could use rimless ammo; it is the extractor for a rimless case. They are spring loaded and retract for a rimmed case.
Pretty smart, these old craftsmen.
