Dear Gary,  Greetings from The Bronx.  The switch in question has a
large cylindrical button on the operator's side, and a rather small
button or stem on the obverse side (behind the front board of the
piano).  What you have to do is check to see if the tube to the valve
of the pneumatic that presses/cancels the switch button from the
obverse side, is still connected.  If it is not, the valve will
operate continuously, causing the pneumatic to cancel the play switch.

A temporary fix is to remove the supply hose to the pneumatic to
disable the shut-off switch pneumatic.  If the switch is actually
broken, and it will not stay locked in the on position, then it will
have to be replaced.  Unfortunately these and many other types of these
switches were not meant to be rebuilt.

One possible fix would be to replace it temporarily with a toggle
switch, you would be able to play a roll with little or no problem, but
would have to manually shut the switch off after the roll has finished
rewinding.  Hope this helps...

D. F. Rowe
Bronx, New York