It's a projector screen. Put it on its side (from the photo) and shine a projector into the mirror. It will bounce off the mirror onto the back of the screen (bottom in the photos) like a rear-projection TV. These were once sold for converting old film movies to video by pointing the projector into the mirror and a video camera at the screen (90 degrees apart).

If that bottom piece doesn't act like a translucent projection screen, disregard this.

It's based on this idea:

https://www.amazon.com/Hama-Telescreen-Cine-Digital-Transfer/dp/B00006IVR6

u/FlokrollLioncour solved it with the comment above. Link to the original post

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