I just wrestled this out of a 1973 Rigarmi NMA after a couple of days soaked in oil. I'm thinking it's an M6x.75, but I don't own anything that takes one so I'm not sure. Anybody know by looking? As an aside, this gun is on the small side and looks alot like some euroarms I've been looking at lately. Anybody with a euroarms around here that has an OAL for their cylinder? An Uberti fits, but has a little endshake.