Basically, its the Dutch equivalent of the Spanish
piece of 8, and likewise used in early Americas colonies as the basis for the dollar
European silver trade coinage, a Ducatoon, its a good
silver at .941 fineness
With this design often referred to as a "silver rider dollar" due to the mounted horseman obverse
shows the dutch coat of arms with the date 1664 above.
These are a "common" salvage from shipwrecks, of which this is probably one.
17th century Pirates loved them.