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.