VAM's as they are known, are die varieties cataloged by Leroy Van Allen and A. George Mallis. You really need to "get the book" on this one to appreciated the extensive amount of research that has been done to cataloge all of the varieties. For MOST, but not all, you will need the book and a good loupe.
Interesting side note on VAM's, especially with Morgan Dollars. In 1997, VAM text listed nearly 2,000 differant die combinations, or more than twice as those listed in their first edition of the above book in 1971.
