Curiously enough, a Drachmann H.C. Bay player is being finished up right
now for a client in my shop.  As with yours, someone "fixed 'er up"
years ago, and peeling back the duct tape reveals what appears to be a
mystery set of expression controls buried mid-point under the keybed!

The keyslip controls were absent when I started on the piano, and the
client wants it to play without his pumping it and sans any expression
controls(!) so, the customer always being right, I am not attempting
to solve the mystery expression controls configuration at this point.

The tempo governor and stack cut-out compartment has extra slider
valves.  These permit the airmotor to run at a faster speed with the
stack silenced (similar to the feature found on original Aeolian
players) when the play/reroll lever is moved to the "silent" position.
They are linked to the regular stack cut-out rod, shunting vacuum to
the airmotor compartment and opening the second upper slider to allow
greater vacuum to flow to the motor while silencing the stack at both
ends.

The biggest challenge in rebuilding the H.C. Bay is getting to the
pouches.  Rather than take the risk of splitting the valve deck and
pouch board to pieces (especially with wood now nearly 90 years old),
I find the following procedure most efficient to use:

Drill a series of vertical holes for a #6 Flathead screw with a
countersink.  (H.C. Bay stack depths vary, so you will have to determine
the depth of the screw hole.)  The idea is to screw the valve and pouch
board together after it has been separated by a thin bladed lengthwise
cut on a table saw.  Use end supports at the beginning and end of the
table aligned to the height of the table saw.  The screw holes need to
be staggered every two notes at the pneumatic vent end, and one wherever
support blocks are at the opposite side.

Once drilled, then separate by the cuts.  Use a very thin table saw
blade, precisely aligning the stack so that a length of stack cut is
made through the front of the pouch/valve chest, then turn it upside
down and repeat the full length cut to the reverse side, then free the
pouch deck from the valve chest.

Replace the wood removed with a thick leather gasket appropriately
punched.  The gasket thickness must be slightly thicker than the saw
blade and/or wood removed to compensate for leather compression when
re-assembled, or you will get your pouch discs raising the valve stems.
Try to leave a gap of 1/64" to 1/32" between valve stems and lifter
discs for atmospheric changes; this distance shouldn't affect the valve
travel speed.

Future rebuilders of that stack will be grateful for the job you did --
taking what is technically a stack never designed for rebuilding and
making it rebuildable for future techs.

Stephen K Goodman