In 1960, as the story goes, actor Robert Stack had fallen in love. Not with a co-star from “The Untouchables,” but with a bright-green 1957 Mercedes-Benz 300SL Roadster.

He would pine for it as he passed it on Sunset Boulevard on his way to work every day. It got so bad his wife Rosemarie finally told him to just buy the car and be done with it. But someone beat him to it.

Unbeknownst to him, Dezi Arnaz, Stack’s producer, had bought the car as a gift to celebrate Stack’s 1960 Emmy for best actor. Arnaz handed the keys to Stack shortly after his win and Stack would own the car for the rest of his life.

VIDEO: Taking a spin in Robert Stack's Mercedes 300SL

Such was the story told at the Petersen Museum on its popular tour of a basement vault housing myriad vehicles rich in historical significance. The Mercedes passed into the museum’s hands after Stack’s death -- he remains the only owner of the vehicle, which has 87,000 original miles.

PHOTOS: 1957 Mercedes-Benz 300SL Roadster

This Mercedes is one of many vehicles the Petersen Museum is now selling through various auctions around the country. The institution has already raised $8.5 million through the sale of about a dozen collector cars. It has said it plans to sell more than 100 cars and use the money to finance a remodeling of the museum’s building, updating exhibits and improving the Petersen’s collection.

This has raised concern among museum experts, who say selling items from a collection to finance projects beyond adding to that collection is a violation of museum ethics. California Atty. Gen. Kamala D. Harris has launched an inquiry into the process the Petersen used to determine which of its vehicles it would sell.

Some of the vehicles are rare or have storied backgrounds or Hollywood owners. The museum has already sold a Duesenberg owned by Bill “Bojangles” Robinson,  the African American dancer and actor who often starred with Shirley Temple in pre-World War II movies

This 300SL will hit the auction block on Aug. 2 as part of Auctions America's inaugural Burbank auction. The event will be shown live on NBC Sports.

The car will be offered without reserve, and is estimated to sell for between $600,000 and $800,000. Such a price would be consistent with pricing on similar 300SL Roadsters, according to a pricing guide published by Hagerty Insurance. The company says that Roadsters with the hardtop can command $50,000 more than those without.

An afternoon outing behind the wheel of this particular 300SL proved it to be an easy drive. The six-cylinder engine pulls strong and smooth, and idles cleanly. It moves the lightweight roadster with gusto.

Meanwhile, the four-speed shifter is light and happy to find the correct gear.

The steering isn’t power operated, so the arms get a workout during tight maneuvering, but otherwise is plenty precise. The steering wheel itself is large by modern standards, and turning a smaller concentric circle within the wheel activates the blinkers.

Though the car was built in 1957, it’s sometimes been listed as a 1960 model, Weaver said. This was because many cars from that era were referred to by the year they were registered rather than their specific model year.

During the 1970s, Stack had the Mercedes painted the maroon color it remains today and had the interior redone as well. Other than some aesthetic improvements to the engine, glass pack mufflers, and a Kenwood stereo, the car is entirely original, down to nearly all of the hoses in the engine, according to Alexander Weaver of Auctions America.

The 300SL has a 3.0-liter, inline six-cylinder engine estimated to make between 235 and 250 horsepower. A notable component of this engine is its fuel injection, a first on a production car and a feature that wouldn’t be commonplace on cars until years later.

Power flows from this engine to the rear wheels via a smooth-shifting four-speed manual transmission.

Though not as iconic as its gullwing-doored coupe version, the roadster actually proved to be the more popular model of the 300SL, with roughly 1,858 built during a production run that ended in 1963.

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