First, let me welcome you to TNet. A question on the weight - is the comma supposed to be a decimal (26.28 grams)? Just making sure that it isn't 28 grams. If it is 26.28 grams, then it wouldn't be suspect accounting for the wear on the piece.
Now for the nitty-gritty (assuming the typo on the weight is correct), it is real and a fantastic specimen. It is a 1653 8 Reale minted in Potosi during the reign of King Phillip IV under the assayer of Antonio de Erguetta. What makes this piece interesting is it is 1 year after the Transitionals. Consequently, there are some minor transitional variations that lingered in the immediate years following. Your piece has a vestige of the PH monogram above the denomination on the reverse which is one of the minor transitional varieties. Not sure of the currency conversion, but I would expect a $350-$400 US dollar value depending on the venue and provenance. (If it was selling with provenance at Sedwick's Treasure Auctions, it might realize $500-$600 for Transitional collectors).