My guess is that its "proof" above 100 proof. That is if its used on a distillate , it reads zero at 100 proof and 20 at 120 proof but that's just a guess. Technically 100 proof is 57% alc but most everyone in modern times would say 100 proof is 50 percent alc.
Best way to test it would be to stick it in Vodka of a known proof and see what the scale reads. I think you will need at least 100 proof vodka or it probably will not sink at all.
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