U.S. 1400, Amadeo Peter Giannini USPS Official Souvenir Page. First Day of Issue June 27, 1973, San Mateo, CA. Never folded, shipped flat.
USPS Official Souvenir Pages are Official First Day of Issue Cancellations, which are a mint stamp or stamps affixed to an 8 1/2 x10 1/2 color page, and include technical and historical information about the stamp or stamps and the designer. The pages were issued flat and unfolded.