This practice began in 1946 and continued for 35 years. It was only applied to coins of 50 centimos and higher, and never to 5 and 10 centimo coins.
The series date represents the date in which that particular type of coin was first minted, and it is the more obvious date on the coin. It is typically (but not always) under the portrait on the obverse of the coin, be it General Francisco Franco or King Juan Carlos I. This date will be 1946, 1947, 1949, 1953, 1957, 1963, 1966, 1975, or 1980. Any change to the coin's design, even minor modifications to the portrait or legend, reset the series date to the contemporary year. If the design remained unchanged, the series date remained fixed as well. For example, the five pesetas coin was minted between 1957 and 1974, but all coins minted during that period bear the series date of 1957.
The mintage date represents the year the coin was actually minted, and it is easy to overlook. It appears inscribed inside a six-pointed star that appears typically (but not always) on the reverse of the coin. The numerals of the mintage year are incuse text inside these six-pointed star, the last two digits of the mintage year (such as "68" for 1968). In some series, another star containing the first two digits of the mintage year (always "19") appears as well.
In 1982, the design date was retired, and Spanish coins began bearing only the mintage date, though some coins in 1982 still used the older two-date custom. If the coin bears the logo of the Royal Mint of Spain (a crowned letter "M") where the six-pointed star would be, then the date on the obverse is the mintage date.
Author: Ryan Hackel ("Cerulean"), 30 October 2011
Image: © Numista