Holes/snaps around rear window??????

[Green chevy c20]

I have a 1965 c20. There are about 20 screw holes around my rear truck window. I thought I was the only truck but I saw another truck that had these same hole. But on this truck there is a snap. For something to snap in to it. My guess is maybe it was a window cover or a shade. Just wondering if anyone know what it is for. I will put pictures on here so you can see what I am talking about.