Oh, that makes more sense than the other ideas I've heard. One guy at the show tried to tell me it was a Finnish capture mark over a German acceptance mark... I laughed so hard I felt bad for the guy, who was under the delusion that the S was interchangeable with the SA and that the W stood for Kaiser Wilhelm... I can kinda see where he was coming from, but still...
Anyway, thanks for clearing that up. I couldn't find a reliable source of info on these markings, so I'm glad to finally figure out what it means (I seriously thought it was a rebuild marking for a while there.) I also picked up a weird box of ammo that's had the cover turned around and stamped with a 1941 on the outside. The clips inside are both different marks, and the ammo doesn't have dirty birds on it, so it's probably not Nazi, but I won't make assumptions.