You have a U.S.Army collar brass there, not a button.
7th Calvary Regiment, Company C
The cavalry insignia was adopted in 1851. Officers and enlisted personnel assigned to cavalry regiments, cavalry squadrons or separate cavalry troops are authorized to wear the cavalry collar insignia in lieu of their insignia of branch when approved by the MACOM commander. Some of the armor and aviation units are designated cavalry units.
If I'm correct in the ID, it's the same outfit that General Custer commander, and Company C was run by Captain Thomas W. Custer at one point in history.
The regiment was constituted on July 28, 1866 in the Regular Army as the 7th Cavalry. It was organized on September 21, 1866 at Fort Riley, Kansas as part of an expansion of the Regular Army following the demobilization of the wartime volunteer and draft forces.
Its official nickname is "Garry Owen", in honor of the Irish drinking song Garryowen that was adopted as its march tune.