What's this thing? Metal sphere with a cut out [solved] (old.reddit.com)

This is really interesting, and the answer thread increases in value because of the additional examples given of lensed versions. Everything about its form makes sense, but you wouldn't just guess it.

Reading the rest of the comments reminds me why I left Reddit though. Every thread ends up like watching Beavis and Butthead outtakes and abandoned reels.