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.)