There were a lot of M42 mounts in the old days. The best way is to find a camera store, screw it on, and see if you can meter. The Mamiya-Sekor Screwmounts of hte period seem to work fine with my older Pentax gear, and were not bad lenses either. Check out KEH.com under their Pentax Screwmount section; there are always a herd of lenses in the normal range.