Rich.
I think what you have found is actually a prewar commercial high/low headlamp switch. These were in common use in the auto industry up until the introduction of the stick control on the steering column for headlight controls.
On the Chev CMP, this switch was installed where you have discovered it to operate the rear convoy lamp that illuminated the rear differential target. If you check the instrument panel, you will see that all lights on the truck, other than the convoy lamp are controlled by the driver from that location.
I think Ford accomplished the same task with a switch box located somewhere at the rear end of the vehicle.
David
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