My London-made Weber Duo-Art grand has the type of transposer that
slides a block across the rear of the tracker bar, so that the note
connections move but the tracker bar looks perfectly normal other than
the transposing lever itself.  This is a fairly common fitting on these
instruments.  Another form of the device slides the note holes as a
movable block within the centre of the tracker bar face.  Clearly, with
a reproducing piano, the simple approach of shifting the entire tracker
bar doesn't work because it messes up the expression coding.  The
4-hole tracking that's standard on later London instruments also can't
support simple transposing.

My piano has an 80-note stack.  When it is transposed on Duo-Art rolls
the end notes play the expression coding! There is no form of pneumatic
cutoff, and indeed none was actually needed with this design.  All that
would have been necessary was blocking the top and bottom note ports on
the static part of the tracker bar, which could be done at the rear,
facing the transposer bar, so as not to be visible from the front.
Maybe this was intended by the designer but missed out in construction.

A similar approach is even easier with instruments having a
remotely-mounted transposer and an 80-note stack -- just don't connect
the tracker bar top and bottom notes and there's no way for the
expression coding to reach the stack whatever the position of the
transposer! Conversely, instruments with 88-note stacks always need to
have the full tracker bar connected and a cutout installed, regardless
of whether a transposer is fitted, and it would all get very complex
for transposed operation.  The cheaper transposer that slides the note
block across the tracker bar face clearly has no way of stopping notes
ports from being activated by expression codes and would need a cutoff
even with an 80-note stack.

The other part of the problem is preventing inadvertent operating of
the Duo-Art dynamics from top/bottom notes in 88-note rolls.  On my
piano, which is a pedal-electric, there is a cutoff pouch within the
expression valve block itself that stops any signal from the tracker
bar from reaching the valves on non-Duo-Art operation.  Instead, a
separate set of inputs to the valve block are activated, these being
connected to the Temponamic lever or (for foot operation) opened
directly from the Duo-Art on/off rotary switch in order to set the
accompaniment level for Themodist operation.  The expression valve
block therefore has two rows of inputs plus a single extra input to
operate the cutoff pouches.  It seems this cutout mechanism is missing
from Bernt Damm's piano, or hasn't been connected properly if it's
present -- it is certainly an obscure part of the Duo-Art tubing.

Julian Dyer