Nhbenz is of course correct. You asked about its age. It was invented in 1880 by H.B. Thompson and patented by him as a "tent slip." The US Army adopted them for military use in 1889. Early ones (made before the Patent expired) were marked with the Patent date. After expiration, anybody could manufacture them without having to pay the inventor a royalty, so they were no longer Patent-date marked. See a Patent-dated one in the photo below, and scans of the original US Patent document, and the Army order adopting them for US Army use in 1889. They are definitely not from the civil war era.