It’s one of those freshnel lenses you glue to your rear window to act as a fisheye lens. It’s actually surprisingly good. I can see the ground about 1.5m (5ft) behind me. I wish it would reach my rear bumper as that’s what I’d like to see but this helps!
Also, I took the picture with a poor man’s smartphone - my apologies.
* fresnel
I remember those from ages ago used for very similar or maybe even exactly the same purpose. One of those analog solutions that went the way of the dodo ever since everybody and everything has a tiny portable computer…
Further nerd-ology: https://en.wikipedia.org/wiki/Fresnel_equations