The above cover is a perfectly genuine New Orleans, La Union Occupation Cover with a USA #65 stamp and dated 2 JAN 1864. The cover is addressed to South Boston, Mass and may very well have been sent by a soldier of the Union Army of Occupation.
However, the large rectangular handstamp at the upper left "Ship Island Military Prison SO. N. O. Provost" is entirely bogus and was added to the cover much later. The handstamp is most likely a rubber stamp (rubber stamps did not exist at the time of the War Between the States). There was indeed a small Union Military Prison on Ship Island, but according to Galen Harrision's research only a small oval red handstamp was used. And there are only two recorded covers with the legitimate Ship Island handstamp. The handstamp on this cover is purely a figment of someone's imagination created for the sole purpose of duping the collector. Taking true genuine period covers (usually inexpensive covers) and adding handstamps or other extra markings is an extremely common way that postal history is faked.