This is one of the great "oops" varieties of the early 1800s. On this reverse, the engraver started punching the denomination into the die, but made the mistake of picking up a punch for a 2 instead of a 5. Once the error was noticed, the engraver punched a 5 over the 2, but traces of the 2 remained under the 5. The undertype can be seen even on coins in grades as low as Good.
The 50/20 variety is slightly scarce, but can be found in most circulated and even Mint State grades. The PCGS CoinFacts Condition Census includes coins ranging from MS64 to MS66.
Possibly acquired by Cornelius Vermeule or Cornelius Vermeule Jr., before 1950 - Cornelius Vermeule III Collection -
Stack's 11/2001):228, $41,400 (this sale was originally planned for 9/2011 in New York, but was cancelled because of the attack on the World Trade Center) - D. Brent Pogue Collection
Louis E. Eliasberg, Sr. Collection - Bowers & Merena 4/1997:1699, $6,600 - Dr. Charles Link Collection