In the 1930s, you could order Cadillacs with custom body styles produced by the coachmaker Fleetwood. They had a brochure with drawings of these body styles, and you could order one by number.
In the 1980s Fran Roxas, a famous craftsman and automobile restorer from Chicago, came across one of these brochures from 1934. Roxas found that two of the styles, numbered 5802 and 5859, had never been ordered, and thus had never been built. Numbers 5802 and 5859 existed only as renderings and blueprints done by a then-Cadillac-designer named John Hampshire.
Roxas, together with buddy and former GM designer Dave Holls, decided to build the cars. From the 1934 drawings.
Roxas and Hall linked up with Strother MacMinn, a design professor at Art Center at the time. MacMinn had also worked in GM's Art & Color department starting way back in 1936, and his design knowledge of the era would come in handy. According to an article in Hemmings,
MacMinn and Holls were able to produce sweeps (essentially French curve templates) and factory chassis blueprints, and created line drawings for the  Sport Phaeton. Fran used a 1937 Sixteen chassis, with its hydraulic brakes, as the foundation of the Sport Phaeton.
As per Hampshire's sketch, the two cars share identical front-end lines, although different bumpers give them remarkably disparate looks.
Fran says the biggest challenge for MacMinn was in designing the rumbleseat in the  roadster, which has both a flush-fitting steel tonneau and a passenger-side door--and it wasn't so easy to fabricate, either. Fran either modified or fabricated all of the exterior sheetmetal (both donor cars had running boards, so the fenders are shaped to work without them). The only original panel in either car is the rear floorpan.
"They're not rebodied cars; they're built from the actual renderings," Fran stated.
After passing through a collector's hands in the '90s, both cars recently went up for auction, fetching roughly a million dollars apiece. Anyone lucky enough to live near Carmel, California, can see the cars in person this August 16th to 19th at the Concours d'Elegance.