It’s a Slide Carousel Tray!
The Slide Carousel Tray is the circular tray holds several (usually 80 or 140) 35mm slides in a carousel slide projector. It is filled with each slide placed in upside down and backwards, so that the image is presented with the correct orientation. The tray has a metal plate on the bottom with an opening about 5 cm wide barely long enough to pass a single slide to the projection gate below it.
