Antique Machinist Made Semi-circuler Protractor with Hand-stamped Numbers

I've rarely met a handmade tool I didn't want to hold on to for at least a bit, and this is a great one. I believe machinist made, a semi-circular protractor, for use measuring angles for cutting, joining, etc. (Though also used as a drafting tool.) Hand-pressed type and etched angles, the metal I think is brass but with a super rich patina, showing a coppery-bronze. C. 1870s-80s would be my guess. 

 8" w x 4" t. Good condition, terrific rich patina, one small patch of oxidation and a bit of greening over the the "60" as shown.