The first Pennsylvania state flag to display the Pennsylvania coat-of-arms was authorized by the General Assembly in 1799. The state coat-of-arms was embroidered on a blue field on both sides of the flag. On June 13, 1907, an act of the Pennsylvania General Assembly standardized the flag's design and required that the field of blue be the same color as the blue of the flag of the United States.
The state coat-of-arms, designed by Caleb Lownes, was that of provincial Pennsylvania's Penn family. Numerous modifications were made to this coat-of-arms between 1778 and 1873, chiefly in the position and color of the supporting horses. In 1874 the legislature appointed a commission to establish an official coat-of-arms for the Commonwealth, and in 1875 adopted, almost unchanged, the coat-of-arms originally designed by Lownes ninety-six years earlier.
The 13-cent Pennsylvania State Flag (Scott 1634) issue displays the emblems of the state seal on a blue field: the ship, plough, and sheaves of wheat; an eagle for the crest; two black horses as supporters; and the motto "Virtue, Liberty and Independence." An olive branch and a cornstalk cross below the shield.
Scott 2005 Specialized Catalogue of U.S. Stamps and Covers
SHG Resources (http://www.shgresources.com/pa/symbols/flag/)