Both the stamp and the cover are genuine -- they just do not belong together. The Patriotic Cover has a period address and a Military Endorsement for the 16th Georgia Battalion. A number of these Patriotic Covers are known hand-carried. This was probably such a cover.
But Patriotic Covers with stamps are much more desireable and much more expensive. The stamp did not originate on the cover and was added to the cover in an obvious attempt to enhance its value. The stamp could have been added at any time from last week to many years ago. It is not possible to determine when this took place. Note how the stamp was affixed directly in the corner in such a way as to make it appear that the postmark missed the cover. Always beware the cover where the postmarks do not tie the stamp to the cover.