Help identifying this.

The works on the bottom look like the type that was used in the Federal beacons manufactured by Federal Sign & Signal Company.
Correct, the heat of the air-popper would rise up to the cubes of butter, which would then begin to melt down through the holes and into the popcorn below. When I was a kid, one of the guys on our town's local volunteer Ambulance Association was too cheap to buy a new dome for his rotating blue light he had on the dashboard of his personal vehicle, and he re-sourced an old blue plastic shoe-polish lid that happened to fit, and used that for his emergency rotating blue light. It looked awful, and when lit, it emitted a grayish looking hue of rotating light and everyone made fun of him for it. I'll bet anything this lens was originally part of an air-popper!