The Porsche 911's great grandfather is going up for auction this summer and it could well be the most valuable car ever sold with the Porsche name on it.

The Porsche Type 64 looks a little like a child's clay model of a modern Porsche. It was Ferdinand Porsche's early attempt at a sports car with the engine in the back. This was the same basic idea the Porsche car company would use in its first car, the 356, and later in the 911.

"If you put anybody with passing knowledge of automobiles in front of that car and asked them what it was, they would probably say 'Porsche,' because it's really pretty obvious," said Leslie Kendall, curator at the Petersen Automotive Museum in Los Angeles.

The car will be put up for auction by RM Sotheby's in mid-August in Monterey, California.

As the earliest Porsche sports car, and with a direct lineage to today's 911s, it could be worth $20 million to $25 million, according to experts at Hagerty, a company that insures collectible cars. That would make it the most valuable Porsche ever sold. There's no record of a Type 64 being auctioned before, according to Hagerty, and, given the car's uniqueness, estimating its value is difficult.

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