You found that at 22" deep, wow all i can say is i would go to a coin dealer see what they say. I found several spanish coins around that time of year at 6"-8" but they all had "copy" on the back of the coin. You might have something there.
If you notice, the example you provided in the second link, has a "CP" on the front of the coin. I assume meaning "Copy" ? The one he posted does not have a CP on the front of the coin. He may have found the second known example of that coin ?
There was one authentic coin struck, and it is in a museum. There were thousands(unknown exactly how many) of counterfeits struck that same year. They have been found from Maine to South Carolina by treasure hunters on another forum. They are "real" but not "authentic US legal tender" as of which there is and was only one. This would be categorized as a REAL PERIOD COUNTERFEIT and one heck of a cool find. Even these sell for a few hundred bucks