A little googling shows you have a plain old GM SM420, the common 4 speed that your truck should have, or no surprise that it does have.
https://www.novak-adapt.com/knowledge/sm420.htm
They say 591665 indicates a 1947-1953 model

•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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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-