I was intrigued by Tom Ahearn's reference to an early Aeolian player
demonstration device.  I would suspect that other player manufacturers
produced models and if there are any setting on someone's shelf,
I would like to have a chance to bid on them.

Several responses have referred to an attempt to show a complete
assembly with a stack cut-a-way and others have referred to how
complicated it would be to demonstrate "how a player piano works".

Like the AT&T TV commercial stresses, with the guy and the kids
("It's not that complicated!"), I think all the device needs is a piece
of an old tracker bar, with a tube hooked to a pneumatic pouch with
a vacuum feed to the pneumatic and vacuum to the tube via a brass bleed
cup.  Then you would need a manufactured pneumatic similar to a Simplex
(with a push rod attached) made out of Plexiglas so the pouch and
valve movements are visible.

With a vacuum feed supplied, you could demonstrate the action as
someone suggested by simply putting your finger on the tracker bar
to duplicate the piano roll hole passing over and the action halted
by the paper covering the hole.  Imagination could do the rest of the
story as it would be evident that the uplift of the push rod plays the
piano.  I see the major challenges are constructing the see-through
pneumatic and coming up with a suitable vacuum supply.

I was also glad to hear that there are others out there that have
thought about making a demonstration device.  Really, the most simple
but yet the most vital item to the whole operation is the little pin
hole size brass vacuum bleed cup and its relation to the size of the
hole on the tracker bar.

I have always been amazed with the ingenuity of the boys back in the
early days for the many ways they came up with doing the same thing but
in a different way, either to improve on something or not have to worry
about patent infringement.  It seems that the thing that they were all
married to though was the little vacuum bleed hole.

Larry Schuette, The Player Piano Man
Raymond, Nebraska