Dear Kurt, Greetings from The Bronx.  Is it the pressure type piano
or the 4-in-1 Wurlitzer unit valve you are not familiar with?  In the
case of the latter, the 4-in-1 unit valve was a later development.
It replaced the single unit valves in not only pianos but in band
organs as well.  (In the case of band organs, not just on the
pneumatic stack but on the snare drum action as well).  I've seen them
used in organs and in both upright and grand pianos.  (I restored a
Recordo baby grand for a customer a number of years ago.)

The valve unit is made of pot metal, which is notorious for
deteriorating.  The instrument I restored had only minor cracks in
a few of the valve units; I sealed these with uncut clear lacquer.
The only other problem I encountered was the gasket which is made of
white blotter paper.  I had to virtually reconstruct the gaskets to
use them again on the valves.

These type valves were used in player actions that had two tiers
of pneumatics; the valve units are situated across both tiers.  An
eighty-eight note piano would have twenty-two of these unit valves,
later pianos would only have twenty as most would only have eighty
(80) playing notes.

Initially these metal 4-in-1 valves were probably more reliable than
the wooden single valve units.  The metal valves were screwed into
a common metal plate (with machine screws), to which both deck boards
were also attached, and traversed the length of the action.  The
wooden valve units, on the other hand were screwed onto a wooden valve
board with either a leather or, in most cases, a rubber gasket.  The
latter was more susceptible to dust and moisture, as the upper valve
facings were usually visibly exposed.

Now in the case of the former, the cabinet piano would have a player
action almost identical to a vacuum operated type.  The only difference
being that the player action would be enclosed in what looks like a
box.  The action would be subjected to pressure on the outside of the
pneumatics, rather than vacuum on the inside.  So the pneumatics would
be forced shut from the outside pressure, instead of being sucked shut
from inside.

This arrangement was more reliable as there was always organ air present
in the unit and thereby negated the need for a separate vacuum pump,
which of course would have only been used for the piano.

Hope this helps,
Sincerely,
Dale F. Rowe
Bronx, New York