THIS IS A SOUVENIR SHEET OF 12 STAMPS AS PICTURED. ~ On May 29, 1997 in San Francisco, CA the USPS celebrated 150 years of US Postage stamps. Historically, the United States hosts a major philatelic exposition in the sixth year of each decade. This tradition was deferred in the 1990s to coincide with the 150th anniversary of America's first postage stamps. The Pacific 97 sheets were available at the Pacific 97 philatelic exposition in San Francisco from May 29 to June 8, or by phone or mail order through the Philatelic Fulfillment Service Center. Although the USPS. originally intended to sell the sheets only during Pacific 97, there were widespread complaints about such limited sales. As a result, phone orders were accepted, if placed before May 29. Mail orders had to be postmarked by June 8, 1997. All unsold stamps were destroyed. This marked the most limited sale of a U.S. postage stamp ever. Interestingly, the Pacific 97 stamps were not printed separately, but rather, were printed side-by-side on the same sheet. The sheets were created on the Bureau of Engraving and Printing's Giori F press, a web-fed combination intaglio-offset press. In fact, one intaglio printing sleeve was used for the entire press run of the Pacific 97 stamps. However, this single sleeve was "chromed" three times, a process which involves using a layer of chrome to coat the sleeve. This layer of chrome enabled the sheet to survive the entire press run. Four offset plates were used to print the enlarged stamp proof reproduction and heading that appear in the decorative selvage of the souvenir sheets. The phosphor tagging block covering the 12 stamps, which is only visible under short-wave ultraviolet light, was also applied at the F press.