There were a few aftermarket manufacturers offering auxiliary transmissions at the time, probably the most well known was from the Brown-Lipe gear company. The Spicer in my truck is a descendant of the Brown-Lipe, from a decade or two later.

1946 Chevy Pictures
1946 Chevrolet 2-ton. In family since new. Splash oiled 235, 4 speed main, 3 speed aux, single speed 6.17 rear. Original GM supplied 12' stake bed, paint on sideboards is still from the factory.
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