What makes someone pay almost $7 million at auction for a classic Mercedes 300SL Gullwing when he could have picked up an equally stunning example at the same sale built the same year and, to the untrained eye, visually identical bar the color, for over $5 million less?

In the case of the pair of 1955 300SL coupes sold at RM Sotheby’s Arizona sale, it was all to do with what was under their glassy paint jobs. The dark-colored car is one of the 1,371 standard steel bodied Gullwings, but one of only two painted in Dunkelgrau gray, and changed hands for a cool $1.71 million. But the silver machine is one of just 29 lightweight cars and persuaded someone to part with $6.825 million to take it home.

Related: Mercedes Takes Stirling Moss’ 300 SLR For One Last Drive Before Parking It In Its Museum

By swapping the stock steel bodywork for aluminum, and every bit of glazing bar the windshield for Plexiglass, Mercedes engineers chopped 209 lbs (95 kg) from the curb weight to create the lightweight SL and help the sports car keep pace on race circuits with exotic European rivals like the Aston Martin DB3S, Maserati A6GCS and Ferrari 750 Monza.

According to RM Sotheby’s, these lightweight cars also featured a tuned version of the mechanically-injected straight six that received a hotter camshaft and higher compression. Rudge center-lock wheels with knock-off spinners were standard (the $1.71m car slums it on regular rims), and you got vented drums up front and tweaked springs and shocks.

This particular lightweight was delivered new to Joseph F Weckerlé, a Benz dealer in Casablanca, Morocco, and was the only lightweight SL sold in the entire African continent. Wekerlé ordered his special ride in Silver Gray Metallic with a blue interior, a 3.42 rear end for maximum camel-beating top end performance and a Becker radio. He sold the car in 1962, but given what it’s just realized at auction, we have a feeling his kids probably wish he hadn’t.

Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable
Ultra-Rare Alloy Mercedes 300SL Auctioned for $6.8M, Makes $1.7M Steel Gullwing Look Almost Affordable