Mamiya DSX 500

From Camerapedia.org

The Mamiya DSX came in two models, the DSX 500 and the DSX 1000. The model designation referred to their fastest shutter speeds, and both used the SX series of Mamiya 42mm screw lenses. SX-series Mamiya lenses have a lens mount lock (unlike previous Mamiya threaded lenses) and a mechanical pin which transmits the f-stop information to the camera, eliminating the need to stop-down for metering.

Link

• Mamiya MSX and DSX, at Ron Herron's "Collecting Mamiya 35mm Cameras"