The United States and its allies suffered from major rubber shortages during World War II as the Japanese had control of most Indonesian rubber tree plantations. To survive without this material, much of the world was forced to settle on a limited supply of synthetic rubber of lesser quality. This material filled some needs but lacked the strength and durability of real rubber. Synthetic tires got half the mileage and this material had limited resistance with contact to solvents such as gasoline.
This created immediate problems with the gas tank grommets on GM trucks. (This rubber ring seals the hole around the large gasoline add-pipe extending from the tank through the right side cab corner.) A synthetic rubber grommet was not practical in this location because of occasional gasoline spills during fill-ups.
A solution to the problem was using a proven material that was readily available in the U.S. It was leather! On the assembly line a punched leather disc was pressed over the gas add pipe and held in place against the body with a metal attaching ring. Four screw holes were punched in the body and ring at the factory for the screws. This leather grommet was not equal to the original rubber unit, but did hold its shape against the elements.
Therefore, you can always identify a GM truck cab from this era because of the four punched holes beside the gas hole. The rubber gas grommets used before the war and after about mid 1946 will not totally cover these four small holes. Unless they are filled, by a body shop, the cab must remain the World War II type with the leather grommet.
This is not a bad thing. The leather fits well and you have a great conversation item on your WWII truck!