Confederate Army couriered mail: dateless/stampless cover addressed to "Captain Wm. Devlin, Com'dg C.S.R. Corps, PATTERSONVILLE, La [Louisiana]", with manuscript directive at lower left "Per Courier" & endorsement at top "Official | Hd. Qrs En'g. Officer | Psh of St. Mary". Opened roughly at right (piece missing); opened on all 3 sides for display.
The cover was sent from the Headquarters of the Engineer Officer in the Parish of Saint Mary, Louisiana to William Devlin, commander of the Confederate States Reserve Corps in the Parish town of Pattersonville; sent as official mail, so stampless.
Per other mail known sent to Devlin, he commanded Company A of the CSRC & was stationed "near" the locale; the unit itself is obscure with no additional documentation available & Devlin does not appear on the register of Louisiana Confederate servicemen. The parish and its key towns were the sites of battles in 1863, and the locale itself was the site of the Confederate victory of 28 March 1863 in the Battle of Pattersonville: it's likely this cover pre-dates this period.
Confederate active service mail was otherwise not 'free': active servicemen (not including members of state militias) simply didn't have to prepay the postage fee - but the receiver had to cover the cost. Torn open at back, mostly complete except right edge missing.