The National nickelodeon #1 tracker bar hole is its version of
expression "Soft".  There are two ways to express on a pneumatic
system: either bleed atmosphere into the stack or strangle the supply
source to the stack.  I know it seems odd but National used a small
hole in the stack that opens and bleeds atmosphere into it in order
to reduce the pressure and lower the volume.

When the system is correctly restored, it works very well as a lower
volume level and helps the music.  However if you have a system that
has not been well restored, using that hole will cause complete drop
out of some or all of the playing notes.

Doug L. Bullock
Alton, Illinois (St. Louis metro area)
http://thepianoworld.com/