This is a very interesting story. The planned mintage was 1000 pieces. When the striking of these medals took place, the dies had "pt" engraved, which indicates Platinum. About 600 of these medals were struck with "pt" on the obverse of a medal clearly made of Gold. When this error was discovered, new dies were engraved with "au" which indicates Gold. The approximate remaining 400 pieces were struck with "au" on the obverse to complete the planned mintage of 1000 pieces.