Well, in short: normally when a country issues a new coin for a certain year, the mint-date will change on the coin.
The mint in Spain worked a bit different. From 1868 to 1982, the large year indicates the year that the law governing the issuance of that particular coin was passed. The little number in the star represent the year the coin was actually minted (See the fourth picture for a closeup of the little star).
So, it could be that you have a 1957 coin that was actually minted in 1971.
After 1982 they started to strike coins like most other countries; with the actual year of mintage.
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