Maybe everyone know this, but, just in case: the original and reproduction dual rear side-lamps for 47-55 panel body trucks came with dual filament bulbs. The '39, etc automobile bulbs most like came/come with a single filament bulb?
As MNSmith states above, you can change a one-filament-bulb housing by replacing the bulb-base. This also is the "cure" for single filament front parking lights (for use as parking and signal).
