Bugatti Aerolithe Prototype
The Atlantic body Type 57S featured flowing coupe lines with a pronounced dorsal seam running front to back. It was based on the "Aérolithe" concept car of 1935. Like the Type 59 Grand Prix car, the Aérolithe used Elektron (a magnesium alloy) or Duralumin (an aluminium alloy) for its body panels. Therefore, the body panels were riveted externally, creating the signature seam.
The production Atlantics (just four were made) used plain aluminium, however. But the dorsal seams were retained for style, and have led to the car's present fame.
Only two of the cars survive. One is in the collection of Ralph Lauren, the second was owned by Dr. Peter Williamson, and won best of show at the 2003 Pebble Beach Concours d'Elegance. Williamson's car (#57374) was sold for between $30 and $40 million at an auction in May 2010 to the Mullin Automotive Museum located in Oxnard, California.
Source: Wikipedia, 2014