Hi all.  I, too, have seen such a tracker bar on an Aeolian player.
I was told by the owner, the late Dick Cox (I'm sure that many UK
readers knew him), that it was a very short-lived auto-tracking system
which used the note perforations in the centre of the roll to sense the
roll position.

I did not take too much detailed notice of the tracker bar but I seem
to remember that there were two very narrow slots between each of the
central sounding holes and this does suggest that if the roll were off
to one side, the roll perforations would uncover the appropriate narrow
slot to give a signal to the appropriate auto-tracker valve.

The tracker bar must have been much more expensive to make but at least
the system ignored the oft-damaged roll edges!

Cheers
Brian Cuff