I've had this tool since the 80's but I've never found the exact application for it. I did find out it was some kind of transmission adjustment tool but never found how it's used.
It is for adjusting the rear band. Hold the small square end in a vice and with a bar through the hole at the other end you should be able to turn until you feel the wrench override, 10 ft lbs. When adjusting on an actual transmission you would attach to adjusting screw, tighten till override then back off adjuster 1 and 1/2 turns as instructed on back of wrench.
I knew a bar went through the holes in the body of the tool but I didn't know about turning it in vice nor what band it adjusted. Thanks for the info.