Inside the casting attached to the rudder blade there is a plastic screw; you have to access it from the bottom.
Having an '82 H14 turbo myself, I would guess that the plastic is hard as stone, won't move a hair if you try to use a screwdriver and the slot will just wear out.
There is a helpful "procedure instruction"
available on the web:
Works great, just did it a month ago. In order to remove the old plastic bolt, I first drilled it out being carefull not to damage the threads, and then used a small propane torch to burn the remains out of the threads.
Good luck !