Beginning in 1976, and continuing over the following twenty years, Mint engravers modified the design a number of times. Quarters were struck at the West Point Mint between 1977 and 1979, but they bore no mint mark. The Philadelphia Mint's mint mark "P" was used on coins struck at that facility beginning in 1980. Coins dated 1982 and 1983, both from Philadelphia and Denver, command a large premium over face value when found in near-pristine condition. Beginning in 1992, the Mint began selling silver proof sets, including a quarter struck in .900 silver; this has continued to the present day. Although President George H.W. Bush signed authorizing legislation for these pieces in 1990, coinage did not begin until 1992 due to difficulty in obtaining sufficient coinage blanks in .900 silver.