Closest I can get at this point - thought first it might be a bale seal or token but note the crowned columns - so I believe it is Spanish - 4-maravedis. Carlos & Juana 1544-1564 Santo Domingo Seville Mint.
So, how did a 16th C Spanish coin end up in a cornfield on the Eastern Shore of Maryland? Brought by a slave from Dominica perhaps? Cool find!