Massachusetts "Oak Tree" Sixpence are comprised of the following die varieties:
Noe 15 Noe 16 Noe 17 Noe 17.1 Noe 18 Noe 19 Noe 20 Noe 21 Noe 22
Sources and/or recommended reading: Noe, S. P. (1943). The silver coinage of Massachusetts. New York, NY: American Numismatic Society.