I wouldn’t have known to look for this, but I happened upon it and now I’m pretty obsessed with it. Visually, I find it a perfect thing, inky red cross outlined in dark blue at one side and target shaped black ink cancellation over the stamp at right, which looks very much like the image of snake to me, tongue extending. This red cross was the logo of the 6th Corps 1st Division of the Union army during the Civil War, and with a bit of research I have learned that the 6th Corps had the most beautiful, geometrically shaped logos, different for each division. (I'm feeling rather compelled to start hunting for examples of all of them.) To the right of the cross is stamped a 1863 copyright by Magee printing house in Philadelphia, which is interesting to me for the fact that 1863 was also the year the actual Red Cross was founded, adopting the Red Cross as its logo at the Geneva Convention that year! It looks to me that this cover was postmarked 1865, mailed from Washington DC to Abington, PA. From what I’ve found, this is a pretty rare one, and what a piece of history.
5 7/16” x 3 1/16”. Cleanly slit open at one side to remove the letter inside long ago. Scattered stains as there should be, all to the good I think.