"...Entitled, "A Guide to U.S. Design Numbers (USD), 1847- 1947," the new collection-building structure focuses on 435 regular-issue U.S. postage stamps that were issued from 1847 to 1947 and intended for use on letters or parcels. PSE has produced a full-color, 56-page booklet with photos of the stamps and a cross listing of the USD numbers and their corresponding numbers in the Scott Standard Postage Stamp Catalogue. The Scott numbers are used with permission of Scott Publishing Co., a subsidiary of Amos Press, Inc..."
Source: www.psestamp.com
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