Go ahead and load with the .321 170gr 32Spl bullets. It works fine, and for many years here in the USA it's as close to "J" bore bullets as we could get. ALSO: if you slug the bore I'd bet dollars to dog turds that it's .320-.321. Earlier between wars guns were made that way on purpose.

Ammo was not readily available, and a lot of hunters/shooters used what they could get. That meant military .323 8mm "S" ammo.You'll encounter an occasional JR drilling that has a small spring loaded extractor milled into the rimmed case extractor to pull out rimless ammo installed into the edge of the main extractor. The .320+ bores were the compromise, allowing for use of any 8X57 ammo, except for the 8X57R-360, which is entirely different.

NICE vintage drilling you have there. The Antinit steel is a premium material and is somewhat corrosion and rust resistant. It rates a two-cool!