What is that thing? That thing is the Rolleimeter rangefinder attachment for the f3.5 Rolleis. If you have not tried one it seems pretty weird but it is a neat accessory. It gives you a rangefinder that it is extremely bright and works great. It is really fun to see a rangefinder floating in air with no optics.
The unit works great and is lightly worn. The rangefinder spot is clearly visible.