Awesome find!
According to Wikipedia, The Grand Army of the Republic (GAR) was a fraternal organization composed of veterans of the Union Army who had served in the American Civil War. The GAR was among the first organized interest groups in American politics. It was succeeded by the Sons of Union Veterans of the Civil War (SUVCW). The GAR was founded by Benjamin F. Stephenson, M.D., on April 6, 1866, in Decatur, Illinois. It held an annual "National Encampment" every year from 1866 to 1949.