[ Ref. Chris Grant in 180909 MMDigest ]

This is the short-lived tracker system that removed the need for
edge detecting ears.  The slots are tubed together as left and right
detectors and fed to the conventional pair of roll shifting bellows.

The idea is that there will always be a few notes in the centre section
of the tracker bar so if the roll moved slightly sideways, these notes
will cause one side or the other of these sets of slots to open and
hence control the tracker.  This design was obviously not as cheap to
produce as the more-often-seen edge holes, either singly or in pairs on
either side of the tracker bar.

Nicholas Simons, GB