1798 FM 1 Real Mexico Silver Old Colony Treasure 1St U. This type is one of a series minted in Mexico City during the reign of Charles IIII. The Mexico City mint. Opened in 1538, is still in operation. In 1783, The American Revolutionary. War was won against the British with the help of similar coins. Spain provided cash in the form of these very same Silver. The American patriots to support the war effort. Weight: 3.30 Grams, Fineness. Diameter: 21 mm, Mintage: Unknown. Thank You for looking. Be sure to add letsshop626 to your favorites list. Check out our other items.