Oddly enough, the design of your coin appears to be that of a Liberty Head Double Eagle.....That's a gold coin made from 1849-1907. But yours appears to be silver and dated 1776...Can you describe (and take better pics) of the coin? Are they any "Replica" or "Copy" marks on it?
I can get a better picture for you if i can. I cannot tell if there are any replica marks on it, ther is just a lady on one side with 1776 and on the other it has an eagle emblum with the words Twenty D