The M4 is the rarest of the already rare Nazi Enigma machines, which could help explain why a working model just sold for a record $365,000 at an auction Wednesday in New York. The Guardian reports the 70-year-old encryption machine was purchased by an anonymous private collector. “The Enigma machine is an exceptional encryption device, one of the most sophisticated and complicated of its type,” a specialist at Bonhams auction house tells the Guardian. According to Gizmodo, the M4 was introduced in February 1942 and played a big part in the Battle of the Atlantic. It was called the Shark-key by US codebreakers and remained unbroken for nearly a year.
Of the 50 Enigma machines currently in museums, only seven are M4s, the Guardian reports. The M4s were used exclusively on German U-boats, 70% of which sank toward the end of WWII, accounting for the machine's rarity. In addition, Nazi captains were told to break their Enigma machines when captured, according to Gizmodo. (Read more Nazi stories.)