I expect this is some kind of calibration knob for a female 40-tpi thread that surrounds an adjustment shaft for the set screw to hang onto. It might be used to adjust a seater stem or a powder measure or any of a number of things.
As a knob, the knurled ring probably works as a jam nut to fix the position of the larger ring or drum at some point of calibration. If there are 25 graduations on the drum, then, once jammed so it turns the whole thread, they represent 0.001" per graduation of depth change into whatever female thread the other end engages.
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