> I believe that the stylus was electroplated (nickel plated) in
> position, and for some reason the nickel forms a compressive grip
> on the stylus.  Does this ring any bells with anyone else?

Nickel is one of those odd metals that plates under tension.
Edison found that the diamond for the Diamond Disc stylus could be
held in position given a light dusting of conducting graphite then
electroplated with the nickel.  As the nickel went on it would pull
the diamond down and hold it quite strongly.

After so many court fights over patents Edison began to just do
things secretly and not go for the patent so it ended up as one of
those little Edison secrets.

Best wishes,

Al Sefl