To Find Fakes, Vintage-Car Sleuth Hunts by the Millimeter.  In 2013 alone, classic-car values increased 28 percent, according to the Luxury Investment Index maintained by London real estate brokerage Knight Frank LLP.

Many sought-after models, such as a 1955 Aston Martin DB2/4 or a Ferrari 250 Testarossa, have seen auction prices triple or quadruple during the past decade. As prices have soared, so, too, has the number of fakes.

A car’s coachwork — the distinctive design and shape of its exterior — is what makes it recognizable to a layperson. For an appraiser such as Schroeder, however, the key to identifying a classic car is its chassis. And it was to the BMW’s chassis that Schroeder returned.

Schroeder knew that on an authentic 328, the gearbox attaches to the chassis three centimeters behind the first set of crossbars that create the chassis’s signature A shape. The attachment point was exactly where it should be. But as Schroeder felt for the threading used to bolt the gearbox to the chassis, he discovered something strange: It wasn’t there. Instead, the gearbox had been welded in place. “Aha! There is something wrong!” he recalls thinking. Schroeder had just uncovered another fake.

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