Sorry guys, but weight from spanish maravedis from XVII century can vary a lot, weight is not conclussive. This one has been cleaned on a quite aggresive way, maybe that is the reason for its color.
I'm a collector of Spanish copper coins, and I can't see nothing which drive me think it is a fake. There is also a bunch of contemporary counterfeits, same interest for me than authentic ones, but I would bet your coin is authentic.
A VIII maravedís, Felipe III or Felipe IV, Madrid mint, circa 1618-1626 (Felipe III 1618-1620, Felipe IV 1621-1626)