These are the taillights on a 1947 Cadillac.

They actually hid the fuel cap. The top part flips up to reveal it.

The two red panels on the bottom are, of course, made of glass; this was 1947.

However, GM design chief Harley Earl wanted to do something different for the 1948 model. His directives would create a new style feature and would require a then-radical material.

For the overall form of the car, Earl was inspired by the Lockheed P-38 Lightning, a WWII fighter plane with an unusual "twin-boom" design. (There is no central fuselage; each of the two engines was housed within its own boom, which provided the necessary volume to accommodate the bulky turbochargers demanded by the performance specifications for the P-38. The pilot sat inside a stubby nacelle between the booms.)

Image: CindyN - CC BY-SA 4.0

GM designer Frank Hershey was tasked with developing P-38-inspired design studies for the '48. These were the original sketches:

Here's a render:

And the production version:

This was the first time that tailfins appeared on an automobile. Now take a close look at the taillights:

Image: sv1ambo - CC BY 2.0

Image: sv1ambo - CC BY 2.0

That shape, in the 1940s, was impossible to affordably mass produce in glass. Thus Earl and his design team turned to acrylic.

(The taillight still hid the fuel cap, too.)

So the 1948 Cadillac wasn't only the first car to sport tailfins, but the first to feature plastic taillights. Nowadays, of course, all cars have them.

Don't say you never learned anything here.

Plastics News reports that the Society of Plastic Engineers is giving the acrylic 1948 Cadillac taillights their Hall of Fame award for 2023.