A 1955 Mercedes-Benz 300SL with alloy fenders, expected to cost up to $9 million.

This 1955 Mercedes-Benz 300SL Gullwing has something special under its silver paint, one of only 29 Alloy Gullwing coupes to be auctioned by RM Sotheby's in Phoenix on January 27. The car is expected to sell for between $7 and $9 million.

Officially known as the "Leichtmetallausführung" or "Light Metal Version," the Alloy Gullwing replaces the steel bodywork of the standard car with aluminum. All glass other than the windshield was also replaced with plexiglass, for a total weight reduction of 209 pounds

The 3.0-liter inline six-cylinder had a competition camshaft, higher compression ratio, new throttle butterfly valves, recalibrated fuel distributor The output was increased to 215 hp. The alloy gullwing also featured sportier suspension tuning, vented front brake drums, and the SL's signature rudge wheels.

Mercedes built the alloy gullwing with the privateer racer in mind, but the higher premium over the standard SL meant few buyers; only 24 were built in 1955, and five more were added in 1956.

The car being auctioned is chassis number 5500332, the 13th of 24 built in 1955. It was ordered by Joseph F. Weckare, an authorized Mercedes distributor in Casablanca, Morocco. Weckare specified several options, including a 3.42 rear axle ratio, a 270 km/h (167 mph) speedometer, and a Becker radio.

In 1982, the car was acquired by Hyatt Cheek, then president of the Gullwing Group Owners Club and director-at-large of the Mercedes-Benz Club of America National Board of Directors. Despite its rarity, according to the auction listing, Cheek drove the Gullwing regularly and took it on many long-distance tours. According to the auctioneer, it is one of the few examples that currently retains its original alloy body and engine, hence the seven-figure pre-auction estimate.