It may date just a bit later. According to McGuinn & Bazelon's book on backmarks, and also to Tice's Dating Buttons, all of the "Wanamaker & Brown" marked buttons appear to be post-Civil War. The firm of Wanamaker & Brown was founded in Philadelphia, PA in 1861 by John W. Wanamaker and his brother-in-law Nathan Brown. They made both civilian clothing and military uniforms. Brown died in 1868, and in 1869 the company was renamed John Wanamaker & Co.