Which Interarms pistol do you have? I have both but the German type is buried too deep in the safe to dig it out right now. The Swiss type receiver ring is about 3mm longer than the one on an S/42, so you are right, barrels would not interchange.
I did a bit of looking up via Google, and found nothing on this, but note the arguments are still raging about those "witness marks", with the usual experts spouting the usual garbage. I once was a member of the Luger forum and told them exactly when those marks were made and why and how they were used. I actually was threatened by an "expert" who was so deep in his own BS that he wasn't even rational. (He also told me, from his superior knowledge, that the Luger was a blowback pistol!)
Jim