Here's a guess: Saurkraut crock rock made of glass.
Seriously, it's a thing. Fermentation weight. And there were glass versions just like yours. The "homespun" version was a stone on top of a plate - but about the second time you smash a ceramic plate with a slippery/wet rock into hours of shredded cabbage you'll be ready to buy a proper weight. (Mine are 1" thick ceramic two-piece versions).
That size would be about right for a two gallon crock. The glass keeps the cabbage down in the brine while it pickles and is easy to clean when it's done.