Spain went through periods where they debased the coins used domesticly, but not in the colonies. These were known as billion or vellon alloy, and probably had more copper, iron, or tin added. There was some sort of exchage scheme that operated that worked by trading these for pisatreens from the colonies, which had a higher silver content. Underweight coins were produced at the Potosi mint in the 1640's and this led to a calling in of the old light coins and a reissue using a new design.