From memory, that "stabiliser" unit regulates to around 7 V. Same as charging/running voltage of earlier 6v systems. Some of those had a big metal-cased zener diode inside as shunt and small festoon bulb as non-linear resistor. (Filament resistance gets higher as it heats up).
When beetle fuel gauge overread, common fault is a loose screw which mounts the stabiliser unit. It's critical for the metal body of stabiliser to make good contact with speedo housing, and speedo housing to make good contact to car body. Stabiliser's body is it's third contact.
Fuel Level Match
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