Three Legs, FS-901, the famous and popular error/variety.  A no doubt original example that likely spent no more than a couple of weeks in circulation.  Not strictly speaking a truly rare coin, with 1083 at PCGS, 1454 NGC versus 252 CAC when listed (they made a LOT of 1937-D Nickels).  However, this has long been one of the iconic 20th Century error/varieties, and along with the 1909-S VDB Cent, 1916-D Dime and a few other coins always on the 'hot' list for dealers.  This is a very attractive coin showing few if any marks, nice luster which is not all that common for a Three Legger and attractive color.  Individual coin images by PCGS; I may add some more of my own later.