The Mamiya M645is the first series of 4.5×6 SLRs made by Mamiya. The finder and screens are interchangeable, but there is no magazine back, only pre-loadable film inserts. All the M645 models share the same accessories (finders, screens, lenses, grips and inserts). The lenses and inserts can also be attached on the later Mamiya 645 models.
The M645 1000S (1976) adds the 1/1000 top speed, a selftimer, a depth of field preview button and other details.