Pictured above are two styles of the A-21569 terminal box cover wing nuts. Both of these are forged bronze originals, not brass. Note that the parting lines on the two examples are different.
The originals are threaded 10-32 as shown. The reason for the single wing nut is so that the cover can be removed without tools, and that the nut cannot vibrate off and be lost. It's off-center mass keeps the wing down even if the joint loosens slightly, and it will not rotate off on its own.
In 1931 there were also some zinc or aluminum alloy wing nuts made with a slightly different geometry as shown above. These are seldom found today, and even the bronze originals are somewhat elusive.
Over the years numerous repros have been made, including solid brass and also brass plated pot metal. Those are not original.