Brass, correct Civil War type standard-issue button used on the majority of Federal-issue enlisted uniforms, as well as some officer uniforms. Large quantities of these buttons were used on Confederate uniforms as well.
Made by Waterbury, USA.
Large, approximately 7/8″, used on Great Coats and Frock Coats.
Medium, approximately 3/4″, used primarily on Sack Coats.
Small, approximately 5/8″, used on Caps, Vests, Cuffs, Overcoat Capes, etc.