In fact, Mr. Vorbach said, two cars with his 3-D-printed parts were in the Pebble Beach Concours this year, although he was hesitant to say which ones before the event, for fear the judges would object. “You never know with judging,” he said.

Even more complicated is when the 3-D parts are made by the original manufacturer from the original plans. Many venerable car companies have their own factory-supported restoration businesses, including Aston Martin, BMW, Ferrari, Jaguar, Lamborghini, Land Rover, Mercedes-Benz and Porsche.

Most share the philosophy of Porsche. “The overall mission of Porsche Classic is to keep the cars of our customers on the roads,” said Alexander Fabig, director of Porsche Classic.

But many Porsche parts came from subcontractors that are gone or no longer have the tooling they used 70 years ago. Sometimes Porsche has to turn to 3-D printing. “Reality doesn’t give us any alternative,” Mr. Fabig said.

The bias against 3-D parts isn’t due to quality. “If you look at it from technology, safety or quality, the 3-D part is often the better part,” Mr. Fabig said. “You can argue it is too good.”

It raises the question, then, is a part made from the original blueprints in the original factory an original part? Scrupulous restorers like Porsche mark the new parts as new, but not all restorers may be as forthcoming.