Look through the lens beside the numbered wheel and you see a double-image of whatever you are looking at. Rotate the wheel and the images move apart or together. Move them together, until they overlap and the number on the wheel by the marker shows the distance to the object in feet.
It is an old-school photographic range-finder.
The inside is complex, with a tiny, angled half-silvered (semi-opaque) mirror, springs, and brass fittings. I didn’t dare fully disassemble it even though it needed a good clean.
A nice little piece of precision retro-tech.
I also found this neat little angle-finding brass folding ruler. There’s clearly been a bit of tool abuse at some point – that blank edge looks like it’s been used as a screwdriver.
Again, it took me a while, then I noticed something unusual about this ruler. Can you spot it?