A unique 1955 Mercedes-Benz 300SL Gullwing coupe is scheduled to hit the auction block on April 1 at Mecum Auctions' Glendale, Arizona, sale. Just 1,400 Gullwing coupes were built between 1954 and 1957, but only one looked like this. It's the only Gullwing to leave the factory in Mittelgrun (German for "medium green") paint, which is also repeated...