In the late 50s the Japanese felt confident enough to start making SLRs that significantly differed from the German stuff. Up to that time they were mostly making Leica-copies, and the SLRs tended to come from Exakta. But in 1959 Canon made the Canonflex, a system camera that had interchangable lenses and prisms. But the Nikon F blew it (and everyone else) out of the water, and Canon scrambled to catch up. In 1960 Canon brought out the lower-end RP, which had a fixed chrome prism. But it retained the film-advance lever that was located on the bottom of the camera.
In 1962 they introduced this RM, which significantly reworked the whole concept. They prism was still chrome (instead of black) and fixed, but it was seated much lower in the body. The winder was moved from the bottom of the body to the upper back, where it could be pushed by the side of the thumb. The biggest improvement was the onboard meter (run by a photocell), so you could meter and set the camera without having to juggle a light-meter in another hand.
This was, I believe, the last of the Canonflex series. The camera used a bayonet lens mount, but refined it further to become the FL series when they rolled out the FX. I'm told that the lens mounts are very close but not quite compatible.
I got mine from the same place I get almost all my cameras, a thrift shop. Mine had been dropped (down a flight of stairs, it appears), so there's a nasty dent in the bottom corner and the outer lens ring; and the cover of the photocell is gone and the meter doesn't work. But because it's pretty much all mechanical—it still fires; the advance mechanism works, and the lens focuses fine. If I really wanted to use it seriously I'd have it CLA'd, but since it's mainly a collectable, it'll do well on my shelf as-is.
For more information: Canon Camera Museum