the hole doesn't look threaded but I might be wrong on that. Rather looks like at one time it had a post or something braised or soldered into it(judging by how the patina is quite different around the hole). So I like the idea of oil lamp wick adjuster knob. That being said I can't seem to find one like it.
Edit- forgot to add that the lack of the patent date which is usually on the oil lamp knobs found in the mid/late 1800s might mean it is a much earlier one if it turns out to be a oil lamp knob