I believe this small brass gauge would have been used for measuring pin drill (pin vise) bits; pin drills are like miniature drills, used for precision work by machinists or jewelers, etc. This is a fairly old one, with just the center row of holes stamped: 0-10 in progression, the largest number for the smallest hole. The shape of it when I first saw it made me think of a hornbook (a small paddle of sorts, usually bearing an alphabet, used for teaching; one day I will find an actual hornbook! ) Anyway, I find it quite satisfying just as an object and could not put it down.I also like the 0-10 scale; it could be used now for judging gymnastic routines! 

4" x 1 1/4" x 1/8". Very good condition, great patina, late 19th c./early 20th c. I would think.