The VISOR radi(c)al cam shows us is the earliest version, with the partly knurled knob at the rear. Mine is the same, but sligtly later, with the knob fully knurled. With this knob you can choose a vertical or a horizontal view.
They were made for the first Leicas with interchangeable lenses. They give, like the VIDOM, a mirror image and show black frames for 35mm, 50mm and 135mm lenses. Later there were quite a lot of variants with different framelines for different lenses, but these finders were all killed off by the VIDOM. The VIDOM was a true universal viewfinder that also provided parallax-correction.
It is a nice finder on the Leica I interchangeable with a 35mm Elmar mounted. This Elmar does not need a rangefinder as it generates enough dept of field. The finder mounts really low, so there is not much parallax.
Leica II, Elmar 35mm f/3.5, VISOR, Tmax400.