This nifty cover was sent on September 12(?), 1861, from Washington D.C., to a Dr. J. R. Kelley, of Nantucket, Massachusetts, by his 18 year-old son, William P. Kelley(Kelly; He used two different spellings of his own, last name!), Company I, 20th Massachusetts Infantry! The 20th Massachusetts Infantry had left Reading, Massachusetts, for Washington D.C. on September 4th. Sadly, on March 5, 1865, Sergeant Kelley would die in a Confederate P.O.W. camp in Wilmington, North Carolina.
To continue, as for carmine, Scott #65, it's in excellent condition! The cancel from the Washington D.C. P.O. is pretty fine, but light/faded in some places.
Lastly, as for the envelope, itself, it's in pretty good shape for being over 162 years old. It's pretty clean. The back flap 90% intact. The envelope was opened at the right side, and has thus been reduced a bit. The right side is a bit jagged due to that. Overall, noted imperfections aside, this is still a really neat cover sent from a heroic Union soldier of the Company, "I," of the 20th Regiment, Massachusetts Infantry, William P. Kelley!