I’ve given my 1953 Leica IIIf “red dial” a CLA, or Clean Lube and Adjust and now I need to replace the half-mirror, or beamsplitter in the rangefinder. The beamsplitter gives the double image that moves back and forth as you focus the lens until the two images become one, then it is focused. On older cameras, the beamsplitter, which is a half-mirror, or partially silvered so it is transparent as well as reflective, looses some it’s reflectiveness and is hard to focus.
After getting the top off I was able to get to the rangefinder.