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Solved the rear axle code mystery (see post 2) by using an older version of the parts catalog, but not the A trans code. See if you can find any numbers on the trans case.
Discovered that the transmission in the truck is a Clark 285V per the tag on its side. Must've been swapped out at some time in the past.
The '65 Shop Manual lists 'A' for the 500-1100 series trucks as being a New Process 435 4-speed.