This is a vintage Leica OOZAB focuslide. I did test its basic functionality and everything seemed fine. One of the vulcanite (?) pads on the H bar fell off, so that's in the box ready to be glued back on. The idea here is that you mount your screw mount Leica and then you can slide it between a ground glass focusing screen and the hole through which you can shoot the photo. This is just the thing to bring to an archive to photograph census records if you time travel back to the 1950s.