Ron:
It's a packing gland just like an old sink valve.
The gland material is leather and is probably dried out.
You have to get the leather pliable again - oil or TFE grease works.
Your control probably needs servicing. Complete instructions for this is
posted on our website (beechhurst.com) - go to push-pull controls - then go
to service.
Regards, FX

Ron wrote:

Twenty year old throttle control with friction lock is slipping. Is
there a way to replace the nylon (material?) gripper that the screw
flange presses against that in turn presses against the throttle
shaft?
I could just replace the whole thing but that would be a BIG job. Any
and all suggestions welcome before I do that.