Absolutely certain a bracket or brace to secure the selector cable to the transmission body is missing, I felt it would be a built-in part of the transmission body but it clearly isn't.
I can see too 13mm bolt heads that appear long enough they might have supported such a bracket and they are appx aligned to the securing bolts position on the cable when it is in place.
I have pointed the 'brace' out in the Haynes book. I drew a diagram of what it might look like as a part, maybe I need to have one made. The photo shows the bolts on the transmission where it would attach. the only ID on the transmission now are the number 520 but the ID plate is long gone.



space for brace or bracket, 13mm bolt heads x2




bracket in Haynes




sketch of how a bracket might look.




bracket in Haynes

 

found one for $20, no name but glad its real and not just my imagination

 

Perfect!


An excellent example of diagnostic thinking. The cable housing must be secured in a stationary position. If not, the inner cable and housing will move with erratic, if any, results, as to transferring shift tower motion to the transmission lever.


How, is it done? You thought, searched and found the answer.
A missing part!!!


Kudos.


Carl