The tracker on my 1912 Henry F. Miller piano with the Miller player
action uses one finger on the left side.  Both the upper and lower
spools are moved to the left by a pneumatic until the left edge
contacts the finger.  As long as the left edge of the roll is in good
shape, it will follow the roll no matter how warped it may be.

Since both spools are driven left and right, there is no buckling of
the roll even during wide movements of the tracker system.  I have one
roll in particular that moves the tracker to just about the end of its
travel at the end of the roll, but the roll plays just fine.

The only problem I have encountered is with a roll that has not been
cut properly.  The holes remain uniformly un-centered for the duration
of the roll.  This has never happened on a vintage roll, just new ones
that had not been perforated correctly.  A replacement roll cured this
problem in all instances where it has happened.

The spring tension on the finger that feels the left edge of the roll
is so light that it will properly track rolls with a soft edge without
causing any new damage.  I have had this piano since 1969 and have not
noticed any edge deterioration on any of my rolls (other than that
which was originally present) from this tracking system.  I like the
idea of moving both the upper and lower spools instead of just the
upper spool.

Bryant S. Stott