Found this on Friday while digging in North Central Texas. It is exactly the same on both sides, made of stone-like material, and measures exactly 2.25 inches in diameter, and is .75 of an inch thick.
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Insulator for a rotary potentiometer. Most old car light switches had them for the dash light circuit. I've seen them on variable speed wiper motor switches also, along with other applications. Usually, the automotive applications weren't quite that thick however, so, it might be for some other industrial use.