According to the Copyright Office, U.S. postage stamps issued before 1971 are definitely in the public domain. In 1971, the U.S. Post Office (a federal agency) became the U.S. Postal Service (an independent agency of the executive branch). The new agency's status permitted it to register copyright in stamp images.
Or did it? There is some confusion surrounding the copyright status of postage stamps issued between January 1, 1971(when the USPS was created) and January 1, 1978 (the year the revised copyright law was enacted). Wikipedia, for example, states that U.S. stamps are "public domain if issued before 31 December 1977." Because we are unable to verify the public domain status of stamps issued during this seven-year period (1971-1978), the prudent course would be to assume these stamps are protected by copyright. The USPS has a system established for granting rights and permissions for the reproduction of stamps.