Prototypes by GM
GM built quite a lot of test vehicles. To try out new looks, a new technology or just to test the reaction of the public to a new style for the next generation Corvette. Cars falling under this category were not meant to be sold. Prototypes can be non-functional clay mock ups, like the early C4 design study shown below.
They can also be fully functional vehicles, like the 1954 Motorama showcar line-up shown below. Most often, after the prototypes served their purpose, the order was given to destroy it. This way, a lot of them are gone now. However, quite a few survived because the order was not followed through or the scrap yard secretly kept the car.