I never saw a mechanical one but I would be interested in how they work.
They were very popular for archery 20 years ago. They work with a series of lenses and mirrors. You look through an eye piece and what you see is light coming in through 2 lenses on the front. You adjust a knob until the light from both sides in in sync, that is, you're not seeing double. When they're in sync, you look at the adjustment knob which tells you the distance. I can say from experience that they're nowhere near as accurate as a laser.
