R. A. Lewis
Because of the financial stress of the US Civil War, a June 30, 1864 Act of Congress placed a new tax on photographs. This meant photographers were required to affix a properly-denominated revenue stamp on the back of each image they produced and cancel it by initialing it with a pen. Since no special stamp was created, US revenue stamps were used, and were accepted by the government on cartes de visite so long as the correct stamp denomination was used. This is a Scott #R13 "Proprietary" stamp with value of 2 cents. The amount of tax required for a carte de visite was determined by the cost of the photograph. A 2 cent stamp meant the photo was worth less than 25 cents. Such stamps date from August 1864 to August 1866, which means this handsome unidentified fellow was photographed at that time.