The dimmable lighting switch is described here, in the 1951 Truck Operators Manual.

That rheostat-style light-dimming switch was very common in most vehicles, and was standard on 47-55st GM trucks (among other years).

(read "Description & Operation" in the Shop Manual and also here in the Shop Manual)

1951 OEM gauges had at least one dimmable bulb in each gauge/speedometer assembly/unit.