Little-known fact - when the Confederate states broke away - they refused to return their stocks of U.S. stamps to Washington DC. So the Union Congress demonetized them all and issued a new Series of 1861. Abe Lincoln was in on it, too. Now, any mint U.S. stamp issued before 1861 would be quite a rarity. But if you do happen to have one and want to stick it on an envelope to mail . . . don't do that!