The 1920s Story and Clark player that I had running as a high school
kid in the 1960s has a tracker consisting of a lever whose position
controls tracking of the driven side of the roll chuck.  There's a
friction wheel driving a small winch winding and unwinding a string
which pulls and releases the lever.  The friction wheel is engaged by
the tracker ears.

It's been 50 years since I replaced the original string, perhaps
cotton, with a length of dental floss.  It's been 35 years since the
piano was operational.  I think there's a list of patent numbers
associated with this tracker.  I'll photograph the thing, if you're
interested.

Incidentally, this upright pumper player is available for free, in
Wilmington, Delaware.  It would need a total rebuild, and undoing
everything I did as a kid.  Nothing, except perhaps my refinishing,
is unrenewable.  It will need to 'go away' when my parent's house
sells.

Douglas Heckrotte
Philadelphia, PA 19119