Mercedes-Benz 300 SL

This 1955 Mercedes-Benz 300 SL gullwing coupe has something special hidden beneath its silver paint. It's one of just 29 "Alloy Gullwing" coupes built between 1955 and '56, and it was sold late last month at an RM Sotheby's auction in Phoenix, Arizona, for $6.825 million. Formally known as the "Leichtmetallausführung" or "Light Metal Version," the Alloy Gullwing replaced the standard car's steel bodywork with aluminum. All glass except the windshield was also replaced with plexiglass, for a total weight savings of 209 pounds, though this particular example, bearing chassis number 5500332, had its windows replaced with regular glass at some point.