The Mamiya M645 is the first series of 4.5×6 SLRs made by Mamiya. It has an electronic focal plane shutter. The finder and screens are interchangeable, but there is no magazine back, only pre-loadable film inserts. This made it easy to preload onto less expensive film magazines, but no mid-roll change is permitted. Up to 15 frames can be captured onto a 120 roll film. All the M645 models covered here share the same accessories (finders, screens, lenses, grips and inserts). The lenses and inserts (but not the viewfinders) can also be attached on later manual-focus cameras such as the M645 Super.
The M645 1000S (1976) adds the 1/1000 top speed, a self-timer, a depth of field preview button other details. The manual advance winder knob with fold out crank is now primary just a crank. It is still removable, so many cameras on the used marketplace might be different or they were lost because some hand grips require them removed.
The M645 J (1979) is like the original model with shutter speeds from 1s to 1/500 but with the mirror lockup feature removed. The J is for Junior. It also lacks the upper shutter-release button of the other M645 models. It was only made for export.