M37 screwmount lenses were among Pentax's first SLR lenses designed for Asahiflex SLRs. These lenses feature a 37mm screw-in mount. They all have fixed focal lengths, and manual focus and diaphragm control. They can be adapted onto modern SLRs using a M37 to M42 adapter together with an M42 adapter for your specific camera mount. The M37 to M42 adapters are hard to find, however.