Thanks for your welcome.  It is some number of years since I seen the
1914 Steinway player piano (88-note), but I recall that there were a
series of slotted holes in about the middle section of the tracker bar,
just above the music holes in the bar.  I seem to remember that the
slotted holes were fixed between the note playing holes, so I assume
that when the roll mistracked the music note hole would admit air to
the tracker bar slot holes.

There were two lots of holes, one series on the left of the centre of
the bar and one to the right.  Sorry that is all I can remember of the
system.

Regards

Brian Lackie
Australia