I've been a mechanic most of my life.
I've seen this problem on EQ mounts numerous times.
Other than someone cross threading this aluminum piece, which
would not be easy to do (due to coarse threads) that "set screw"
is likely the cause of your stripped thread grief. I strongly suspect
that in this scope's history (perhaps when it was first assembled)
this set screw was tightened a bit too much and it buggered up
the steel shaft threads. If the set screw was overly tightened
(or even semi-tight) it could have become mushroomed. That
would have made it difficult to unscrew far enough to prevent further
thread problems to the steel shaft while attempting to remove the
So... when the aluminum piece was eventually unscrewed, the still
in place tight set screw, would further damaged the steel shaft.
Then, when the aluminum piece was re-installed, it's threads were
likely damaged even more due to the steel shaft's now damaged
threads. Eventually the aluminum piece became fully stripped.
Been there, done that. Seen it. Fixed it many times.
THE FIX :
1: Heli-Coil the stripped aluminum piece.
2. Make sure the set screw doesn't interfere with the Heli-Coil threads.
Either unscrew the set screw out far enough, or ground it down so
that it doesn't cause future problems.
3: If the steel shaft is too "screwed up" you might consider buying
a thread file. I've used em' for years. The can, and have been,
invaluable parts savers for me on many occasions. Very very useful:
Finally: "Screw the steel set screw."
Now use Locktite to hold the aluminum piece in place after adjustment.
Works like a charm and no more stripped threads.