Anybody know what this antique tool is?

Hello,
It looks like part of a early indicator. You might go post it over on the Antique Machinery and History section of the,
http://www.practicalmachinist.com/ubb/ultimatebb.php

I know what it is meco, I use it occasionally, and you are close.

Do the numbers correspond with degrees, in a circle?

Mike, you have problably used the modern version quite a bit. LOL

With a smaller lock screw, it could be a windage adjustment for a gunsight. I will say it is for machining tool of some sort but not my area of knowledge.

its thr predessor of the modern dial indicator

BINGO, We have the correct answer. The tool and its modern day counterpart, some call it the last word indicator.

Well you got me in. What the dickens is a dial indicator. What does it indicate

LOL,Strine. The old indicator is graduated in thousandts of an inch And new version in the background uses a rotary dial graduated into half thousandts. Its used by machinists to dial or locate an exact point on an edge etc