I found this is at the house of a deceased relative (IOW asking the former owner is not possible for obvious, sad, reasons).
I slid a "Get out of jail free" card into it to illustrate what I guess it was used for (holding something thin, but what?).
Putting the 2 thin cardboard pieces back was not so easy, so I wonder how this was mass produced (it doesn't look like a one-off piece to me; the cuts are very clean and parallel).
It was with a pile of papers, estimated to be at least 30 years old.
Searching the net didn't give any leads, but that could be my poor google-fu (though I saw a lot of interesting origami projects in the last hour LOL).