Look for a tag on the auxiliary, probably on the left side.
My first guess is it is a Watson brand 2 speed. But until you uncover more of it that's just a guess.
This is a Watson direct and over http://rides.webshots.com/photo/2922716010080251109JoUrvp

A Spicer 5531 looks like the 5831 that replaced it, and yours doesn't appear to be either.

I haven't seen enough of the old fullers to know if that's it or not.

Grigg

•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-