Maybe fake, or just heavily corroded from salt water exposure. Many of these are sea salvaged and the silver reacts with the salt, unlike salvaged gold coins.
Looks interesting, maybe someone tried to fake a sea salvage. The rimming seems strong going into the fields on the obverse around HISPAN . ET IND which looks very bad. Can we get a clearer picture of the reverse?
Alright looking over it again, I would say fake. The lettering is too thin and the ET in HISPAN . ET is a give away. image of a genuine from University Of Notre Dame's collection
See this is hard, the thin letter is most likely from sea salvage from looking at these images because this def looks like a sea salvage. grrr
if this is a sea salvage and the coin is from 174?, there was a total of 7 shipwrecks spreading from the Bahamas to the Netherlands so I don't think your gonna find the shipwreck it came from.
It has the look of a sea salvaged coin, but at 21 grams, that much corrosion would render it basically a metal slug without much (if any) readable details. It's just a modern novelty replica.
Such items, even closer to normal weight, are impossible to authenticate due to the extensive surface damage. If it doesn't come with a photo certificate or isn't encapsulated by a reputable TPG, it will always be suspect (if not by you, then by anyone you try to sell it to).