A few years ago I rebuilt a player with an H. C. Bay stack in it.
I do not remember the brand of the piano and am too tired to look it
up in my records.  It had the "mystery holes" through the keybed
as well.

After a lot of head scratching, I determined that the original design
called for the key stop rail to be engaged in the usual manner from
under the keybed.  There were cutouts strategically placed so that
three of the sharps would continue to function in a downward motion.

When the stop rail was engaged three linkages also were pivoted into
place under the appropriate sharps.  They would then transfer the
movement of the sharp keys through the keybed to strategically placed
pallet valves which would trigger the typical expression pneumatics
found in upright players.  It seems only one of the original transfer
levers was still in the piano.  I used it for a pattern to manufacture
two others.

I had a great deal of trouble getting this system to work consistently.
It would either fail to engage or worse, fail to disengage and the
sharp keys would not be able to function when manually playing the
piano.  I finally made some simple transfer levers that the operator
could work from under the front of the keybed to operate the expression
pneumatics.  It was an interesting concept but like a lot of things
that were tried, I am not too sure of its overall effectiveness.

Norman Cantrell