One of the many smart things Nikon did when they designed their F camera was to give it interchangable viewfinder prisms. The early cameras had a plain prism with no integrated metering system. They later created their Photomic finder which had a CdS meter and measured through-the-lens (TTL). But before CdS became available in the early 60s, they had this clip-on version with a selenium cell. The cell was too big to use TTL and you have to take your eyes away from the viewfinder to read the meter. So you don't see these often. The very few times I have seen them they're sold separately. Almost all F bodies you see these days come with Photomics.

Paid about $7.50 from eBay. In-transit, so a photo will be posted when it arrives.