Ok, I know this isn't made for Leica, but it fits in the shoe and it works. I've got the gist of it - it's an old Kodak Signet 80 multiframe finder with 35mm, 50mm, and 90mm bright lines. However, what do the "3" and "5" at the top of the 50mm bright line mean?
Why, I'd know those lines anywhere... They're parallax indicators.
Supposed to help you keep from cutting off people's heads when you are shooting at 5 feet or 3 feet.
But I'm rarely able to "remember", in the excitement of taking pictures, to refer to that line and so... Unless the rangefinder has automatic parallax correction, I frequently cut people's heads off.