Here's what I found about your buttons manufacturer. From the book "American Military Button Makers and Dealers; Their Backmarks and Dates" by William F. McGuinn and Bruce Bazelon:
State Arsenal, Harrisburg, Pa. Made uniforms for the National Guard of Pennsylvania, ca. 1898-1902. Buttons with this backmark were ordered from Scovill and Waterbury Button Co.for these uniforms. This backmark is found on PA-22C & D, also PA 20 & 21.
The numbers refer to the listings in "The Record of American Uniform and Historical Buttons" by Alphaeus H. Albert. Your button is PA 22C. It is listed in his book as follows:
PA 22 Pennsylvania, the state seal; 2-piece, low convex.
A. The Device.the eagles wing close to the horse's ear.
C. The wreath with five twists. "State Arsenal:/ Harrisburg. Pa." RV 1.
The RV stands for relative value And the -1 means it is a fairly common button. RVs go from 0 to over 250.
It still a very nice find and the first one I have ever seen.