At least they're consistent with their mistakes. lol side note: I get messed up if I'm trying to thread something under a sink while using a mirror. My brain couldn't get around it. Had to rely on muscle memory
Media mills - typically a large cylinder filled with these balls and a product you're trying to size reduce...say like a lumpy pigment or other powder. The cylinder rotates and the balls break up the lumps through contact and attrition. Over time the balls wear out (become out of round) and...
Hollow? Maybe ingots or some type of glass form that made it to this point but never finished. I don't know enough about blow in mold bottles, or light bulb production but thinking about something along those lines.