To start with, it is not an "RF" (RangeFinder) camera. It is an early 35mm SLR (Single Lens Reflex).
It appears to be a Praktiflex model. It look similar to the Praktiflex FX, but is marked "M-X" and lacks the flash sync sockets to the right of the lens mount.
Here's the Camerapedia page on the Praktiflex FX for comparison:
http://www.camerapedia.org/wiki/Praktiflex