1793, 1794 or 1873 (including all the open and closed varieties) Harry X Boosel (AKA Mr. 1873) made the 1873 research famous with his book 1873 - 1873. I'm still looking for a copy and have it listed in my wants page here at CCF, I do have a photocopy of it though, and there are so many 1873 varieties it makes my head spin!
"westcoin wants" - Numismatic Books & US Two Cent Pieces
"Buy the Book Before You Buy the Coin" - Aaron R. Feldman - "And read it" - Me 2013! ANA
Life Member #3288 in good standing since 1982, EAC Member #6202, NBS Member, 2¢ variety collector.
See my want page: http://goccf.com/t/140440