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.  Please note that due to value, this will require Registered Mail shipping, which is slower delivery time; please take this into account prior to considering a purchase. 

PCGS 3982.58/50834685