Why couldn't the Corsair's ammunition boxes be swapped at first?

As originally designed, the F4U-1 had a distinct ammunition container for each of its 12 positions: six per wing and two per machine gun. None of the containers was interchangeable with another. If one was damaged, it could only be replaced with an identical, specific spare, which meant keeping 12 different part numbers in stock just for the ammunition containers.

The rear outboard containers — number 2 on the left wing and number 4 on the right — sat in a part of the wing where the airfoil shape and planform taper required dimensions somewhat different from the rest of the containers.

In mid-1942 the Navy's Bureau of Aeronautics asked Vought Aircraft to reconfigure the containers to make them interchangeable, easing some of the logistical burden on squadrons operating the Corsair. Vought complied, and by December 1942 the redesigned containers were already being fitted on the production line. The change involved shifting the ammunition rollers so the container fed from the opposite side. The result left only two container types per side. From then on, container number 1 could be swapped with number 3, and number 2 with number 4, although this standardized arrangement only appeared from aircraft 157 onward.