That car was the Buick Blackhawk, announced late 2000, first displayed early in 2001, and linked to Buick's 100th anniversary in 2003.
The Blackhawk used the grille from a 1939 Buick and some parts from later models, though the design other than the grille was essentially new. I found no single source providing a nearly complete version of the Blackhawk's background, though various aspects are discussed here, here, here, and here.
The Blackhawk's concept theme is that of a "street rod" of the late 1940s, but with 2000-vintage details such as the wheels. Most of the photos below are either factory shots or are from auction houses publicizing its availability -- I'm not sure of most of the sources. The street view of a '39 Buick is one I took.
General view of the Blackhawk. I think it is nicely shaped, in part because stylists were familiar with 1940s design conventions. That is, they simply had to refine rather than innovate. I dislike the 2000-vintage wheels because they strike me as being out of character with the rest of the car.
This view from the rear shows the hot rod inspired dual exhausts. The tail lights and the brake light + badge on the trunk lid are from 1946-48 production Buicks.
A slightly rear-oriented side view. Adding street-legal bumpers would have degraded the purity of the design. RM Sotheby's photo.
The top retracts. Here it is in its raised position.
Finally, a 1939 Buick Special Convertible Coupe. Below, I repeat the image of the Blackhawk in the first photo for comparison.
