We’re all used to the idea that a Mercedes is expensive. Some more than others, of course: a CLA starts at just over $30k, while a Maybach S-Class will set you back more than five times that amount. But $8 million?

That’s about how much the experts anticipate this particular Mercedes to sell for at auction next month. But as you might have guessed, this is no “ordinary” Mercedes.

What you’re looking at here is a one-of-a-kind version of one of the German automaker’s most coveted models: the 540 K Special Roadster. The drop-top grand tourer was the ultimate in luxury transportation in its day, stretching over 17 feet in length and packing a 5.4-liter supercharged straight-eight.

Of the 419 examples of the 540 K that Mercedes produced in the late 1930s, only 25 were equipped as Special Roadsters. The example you see here is the latest example known, and may have been the last ever made before the outbreak of WWII. And it further stands apart with several unique features that make it even more elegant than any other.

The car was ordered new in 1939 by one Rolf Horn of Berlin. An art dealer by trade, Horn had his roadster completed to his superior aesthetic taste with a new number of special touches. The doors are cut lower, the fenders tapered to a teardrop shape, the running boards replaced with sculpted bodywork, the roof folds under a hard deck, the single-frame windshield raked further back, and a chrome strip elegantly ties it all together.

Horn put it in storage shortly after taking delivery as the cloud of war loomed large. Once the dust settled and East Berlin was under Communist control, it was used by Soviet diplomats and was transported to Moscow. It was subsequently smuggled out from behind the Iron Curtain to Sweden, then to the United States where it remains today.

Certified in highly original condition, having undergone painstaking restoration, and decorated by top prizes, this Special Roadster is slated to emerge as the top lot next month in Arizona, where RM Sotheby’s expects it will sell for between $7.4 and $8.4 million.

The Canadian auction house should know, after all, having handled the sale of several other Special Roadsters over recent years – including this very same vehicle, which it sold at Monterey in 2011 for $4.62 million. It sold one in Arizona earlier this year for $9.9 million, but the record for its type currently stands at $11.77 million, achieved by Gooding & Company at Pebble Beach in 2012.

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