Hi All,  The 'mystery hole' is there to prevent the air motor governor
from 'locking up' when the Tempo is put to zero (0).  It's an 'escape
port' that allows atmosphere to enter the governor.  The other holes
are most likely for alignment of the plate.

There are a number of player systems that employ what I call an
'escape port' for the air motor governor.  In a really air-tight
system, when the knife valve is 100% closed, the governor bellows
will close it even further, and even when the tempo lever is moved
away from "0", the motor will not move because the knife valve is
'locked' closed.  To eliminate the possibility of that happening,
the 'escape port' was invented.

The same 'lock up' situation can occur when the pedal pressure is
extremely high and the air motor governor is not correctly adjusted.
To solve that problem, place the tempo lever at 70 and close the
governor bellow by hand.  If the motor stops complete, turn the
limiter screw inwards until the motor just starts to crawl.  That
will prevent lock up from ever occurring.

Musically,
John A Tuttle
Player-Care.com
Brick, New Jersey, USA