Using a 3-wire Turn Signal Switch with Single-Bulb Tail Lights.
Up until the '62 model year, VW Beetles had tail lights with only a single
bulb in each housing. It was a dual filament bulb. The smaller
filament was for the tail lights.
The brighter filament did double duty as both stop light and turn signal.
To enable one filament to be used for both stop and turn, the turn signal lever switch had a complex switching mechanism with 6 wires brought out.
Beginning in '62, a far simpler switch with only 3 wires was used and the tail lights had a separate bulb just for the turn signals.
If, for some reason, it was desired to use the later 3-wire switch with early single-bulb lights, some sort of auxiliary switching circuit would be needed.
The same situation exists in the case of an early car with semaphore signals. If flashing rear turn signals must be added (to meet state inspection requirements), then a similar auxiliary switching circuit is needed.
There are many ways to design such a circuit; I have included a neat version built by Gerry of Trick Engineering & Fabrication here:
You can contact him directly for details. "Gerry"
He will even build one for you in a suitable box at a very nominal cost.