Three Porsche Type 64s were built for the Berlin-Rome 1500km road race, but two were destroyed and that race never took place.

The last surviving Type 64, however, was carefully kept by Porsche racing driver Otto Mathé for years and now, after changing hands only twice in seven decades, it’s for sale at RM Sotheby’s August Monterey auction.

The car was built to satisfy Ferdinand Porsche’s racing itch but also to advertise Germany’s new car for the masses: the KdF-Wagen. The full story is on motorsportmagazine.com but the crux is this: The Type 64 could become the most expensive Porsche to sell at auction, with an estimate of $20 million, because without this, there would be no 356, 550, or 911.