on the side of the transmission it has rasied markings " 5 " "GM" "590474"
on the top it has "E 16 6"
also found a bow tie with a 4 next to it, and the number 590511 on the side.
Looks like your transmission is original to the truck.
E=May
16=16th the month
6=1946
There are some bronze bushings in the 4 speed so make sure you use a gear oil that is good for "yellow metals". Some GL-4 and 5 formulations claim they are yellow metal safe, but it rarely says so on the bottles. GL-1 90 is what I use in my 4 speed.
Paul
