Looking in the 1949-53 GMC models 400-980 parts book the index list auxiliary transmission in section 7.5200.
That section is really for speedometer adapters but list some of those for use with model 6231 and 8031 auxiliary transmissions.

I don't know what that says for the auxiliary being factory installed or not.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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