I restored an Ennis Piano about fifteen years ago.  I don't remember
too many specifics, but the tracker system stands out.  The large
pneumatics do double duty, serving as both tracker system and
transposer.

The tracker works similar to any other hole type system.  As a hole
at the edge of the tracker bar is covered by one side of the roll,
the tracker bar is shifted by the tracker pneumatics in the other
direction, until leakage on each side of the tracker is in balance.

The transposing part comes into play thusly: When the player is in the
normal position, suction used by the tracker is applied only to holes
number 3 in the bass tracker side and number 3 in the treble.  If you
want to transpose up , you open holes number 2B and 2T.  Now, the
tracker system thinks the paper is out of alignment and shifts the
tracker bar one hole tracker bar hole, until the role is in balance
again.  Instead of making micro steps, it makes a macro step and then
settles down again.

Potential problems do arise.  The tubing behind the tracker must be
loose and flexible.  The tracker pneumatics are moving a lot of
stiffness.  To get an idea, try to move the tracker by hand.  If the
tubing is original, the tubing may be rock-hard, like spaghetti.  Even
if it's only a few years old, it may be stiff.  The tubing must not be
so tight such it binds on the back of the spool box.  Each tube holding
down on the cover reduces the tracker's efficiency.

The tracker pneumatics are large but if they leak at all, they may not
be strong enough to do what they are supposed to.

All in all, I think it was a unique idea, but may not work as well in
reality.

Jeff Davis