Ok, Qubie is a '35, not a '39, but she's close. She came with a single left side rear light with two single filament bulbs, the upper is the stop and the lower is running/license plate illumination. No turn signals.
She also had two 4" reflectors in the back. I bought a pair of LED trailer lights and a cheap 6-12 volt converter online. I replaced the two reflectors with the LED lights. Much better running and stop illumination.
I still have no turn signals. I replaced my original stop lamp with an LED light, running off the same 6-12 volt convertor. The running light is still a bulb, as the glass envelope of the bulb is a required part of the design. It seals against the rubber ring to stop water intrusion.
I'm sure that your right light is just a left without a lower socket (stop only, no running). The extra piece is there (as mentioned before), because it's cheaper to just leave it in.
