The OM-1 MD is an all-mechanical SLR. Originally, the bottom plate needed to be modified to mount a motor drive on the OM-1. In 1974, Olympus launched the OM-1MD (MD standing for Motor Drive), to which a motor drive can be attached without need for modification. This new version has a small plate marked 'MD' on the front, and a small slot with a circular cover on the underside that covers the motor drive coupling. It has a large viewfinder with interchangeable screens but a fixed prism. It also has a through-the-lens exposure meter controlling a needle visible in the viewfinder that needs a MRB625 or PX625 battery accessed behind a screw in plate on the bottom of the camera. It has a compact body, essentially retained on later models. The shutter speed dial is located around the lens mount, which allows photographers to keep the camera at the eye between shots more easily than SLRs with the dial located on the top plate. Originally, the bottom plate needed to be modified to mount a motor drive on the OM-1. In 1974, Olympus launched the OM-1MD (MD standing for Motor Drive), to which a motor drive can be attached without need for modification. This new version has a small plate marked 'MD' on the front, and a small slot with a circular cover on the underside that covers the motor drive coupling. Most of the information was gleaned from Wikipedia.