There is a device on the back of almost all Coinola stacks that is
a pneumatic the controls a knife valve in the box it is attached to.
A single small nipple supplies the vacuum to close this pneumatic.
In some pianos this pneumatic closes and also opens a pallet valve.

I was under the impression that this unit acted somewhat like the
Duo-Art knife valve regulator.  Some others are of the opinion that
it is a softening device controlled by the lock and cancel valves,
number 15 and 16, used to control the snare drum.

I have not had a piano with its original tubing so I can only guess
what this devices purpose was.  Does anyone know?

Don Teach
Shreveport, Louisiana