Underwater Viewfinder?

I found this in a box I got from a friend. What is it and does anybody still use it? (or want it?)

Not sure how it works. Would like to learn before throwing in the bin.

That is a viewfinder designed to be used with the Nikonos cameras. It fits on the cold shoe on the top of the camera and the masks are for different lenses to cover the AOV of the lens. The dial on the viewfinder is for distance adjustment. Useful if you are still shooting a Nikonos film camera or to a collector perhaps.

Ok thank you very much. Will try to find a collector, that's better than a landfill.

ebay for cheap may find a home that way.