Cita APA (7a ed.)
United States Postal Service. The Postal Service Guide to U.S. Stamps: Updated Stamp Values (Vigésima octava edición.).
Cita Chicago Style (17a ed.)
United States Postal Service. The Postal Service Guide to U.S. Stamps: Updated Stamp Values. Vigésima octava edición.
Cita MLA (9a ed.)
United States Postal Service. The Postal Service Guide to U.S. Stamps: Updated Stamp Values. Vigésima octava edición.
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