[ Ref. Eric Shoemaker in 180911 MMDigest ]

Eric is absolutely correct.  We came across a number of these unusual
tracker bars back in the 1970s when we restored lots of player pianos
in England.  I think this was a British Aeolian thing.  The idea was
that the twin slots should line up with a note hole, and if they don't,
then shift the tracking device until they do.

Of course there may be no note hole in the appropriate place for quite
a long time, so there had to be a number of twin slots in the hope that
at least one pair of them had a note hole corresponding with it.  They
would have been tubed in parallel, with left slot to left slot and right
slot to right slot.

In fact we never came across one of these still connected -- all
examples had had ears added for tracking, so I think we can conclude
that as a system it was unsuccessful.

David & Lesley Evans