I generally would not rate foreign silver so high, but it is more like Spanish silver: its old, and circulated in the US like a half dime. It was worth about 6 cents, and Mexican half reales are the reason they did not make half dimes from about 1808 thru 1829 (had it been a US half dime from that era, it would be up near #1, as my other is). Its also in half decent shape as these things go (no hole, readable date). Here is the original blog entry
Found: 29-Oct-2012; Location: field; Machine: E-Trac.