1807 50C Large Stars 50/20 AU55 Certification #10839522, PCGS #6086
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.