1970-S Quarter Error (1970-S 25C Mint Error), Overstruck on a Struck Philippine 25C NGC PF64. Brassy PF 25c (Struck on Philippine 25 Centavos). This copper colored proof error 1970-S Quarter Error is unnatural looking because it was struck over an existing coin. The reverse design of the Philippines twenty-five centavo coin appears inverted on the obverse of the Washington quarter. The legend from the reverse CENTRAL BANK OF THE PHILIPPINES is clear and surrounds Washingtons portrait. Some of the obverse details show on the reverse of the Washington quarter, but they are not as pronounced as those on the obverse. The centavo was made from copper-zinc-nickel, giving the quarter its peculiar color.

The United States Mint has struck coins for forty-three other countries beginning in 1876 and continuing through 1983. (In 2000 the Iceland Leif Ericson Commemorative was struck by the U.S. Mint.) The first foreign coin struck by the U.S. Mint was not actually a coin. It was a cent token dated 1833 for Liberia, which portrayed a freed black man standing next to a palm tree with a ship in the distance. In 1874 a law was passed authorizing the Mint to produce coinage for foreign countries according to the prescribed standards and devices of such country [as long as] the manufacture of such coin shall not interfere with the required coinage of the United States.

Error coin specialists always look for coins struck on foreign planchets. However, a much more unusual occurrence takes place when a coin is struck over an already existing coin, as in the present case. Obviously a struck Philippines coin remained in a tote bin and got mixed up with new planchets for the U.S. quarter. However, the fact that it is a proof coin makes this overstrike particularly unusual.

All proof error coins are rare because of the quality control that is exercised during their manufacture. Proof coins are struck by technicians who hand feed the blanks into special presses. They are produced, examined, and packaged using extreme quality control. It is very unusual to find major proof errors. They are aggressively sought after by error specialists and those who are interested in the unusual.