Check the barrels on the underside near the breech. If it says 7.7 or 7.8X57 it's a J bore 318. If it says 7.9 it's an S bore 323.
Most J bores run around .320, some .321, and some tighter, but my experience has been most will slug to .320-.321. It gives a safety margin for those who used what ammo they could find, mostly the S bore. During and after the war not just a few hunters in Germany shot military rimless S bore ammo. It was all they had. They would use a cleaning rod to knock the rimless ammo out.
There is also a gizmo that can be added to rimmed extractors to extract rimless cases. It's a spring-loaded tapered pin that slips over the smaller "rimless" case rim that is milled into the big rimmed case extractor. I had two 7X57R drillings set up this way for rimmed and rimless ammo. I wish I had kept one of them.
