Although I wrote to Jason Fisher giving him my phone number,
I think that a short article may help others who want to know about
the "Half Duo-Art" Weber that he wrote about.

Basically, the first expression mechanism that Aeolian introduced
was the Themodist.  The fastest way to know if a piano roll has this
Themodist feature is to look for the "snakebite" or double-dot hole
perforation at the extreme left and right of the roll.  In all
Themodist rolls, the bass perforation is to the left of note #1 (or
you could call it the note #0 space) and the treble perf is to the
right of the space for note #88 (or you could call it the note #89
space).

The principle of the Themodist is that the air flow from the stack
can be choked or strangled to a level lower than it would be if it were
not going through this so-called strangling or regulating mechanism.
I prefer the term "strangling" to "regulating" as the latter can mean
regulated high or low, while strangling always means reducing the flow
of air, and consequently the piano must play at a lower volume.

With a Themodist mechanism, the operator of the Pianola (or "pianolist")
can not only constantly vary the volume of the music, but more importantly
can vary the levels of the Theme and Accompaniment.  The maximum effect
can be obtained by strangling the normal or Accompaniment suction down
as far as possible, so that these notes play quietly, while vigorously
pumping the pedals so that the notes that play as the snakebite perf
goes over the tracker bar are played as loud as possible.  These are
the Theme or Melody notes.

In other words, the Themodist allows the pianolist to bring out or
emphasize or play louder the Melody notes.  When any human plays the
piano with their fingers, they either consciously or unconsciously play
the Melody louder than the other notes (the Accompaniment).  One thing
that must be pointed out is that it is totally within the pianolist's
control to choose the volume of the Theme _and_ Accompaniment at any
instant.

All that the Duo-Art system does is to assign fixed levels to the
Theme and Accompaniment, and the old Themodist mechanism does the rest.
Once again, the greatest contrast in playing would be obtained by the
Accompaniment accordion not collapsing too far, so the normal or
Accompaniment notes would not play too loud, while collapsing the Theme
accordion all the way so that the Theme notes (the notes that play at
the instant a snakebite hole is crossing the tracker bar) play as loud
as possible.

Finally, let me explain what a Half Duo-Art does.  On this type of
piano, the pianolist is playing a normal Duo-Art roll, and the coding
is telling the Accompaniment accordion how far to collapse.  This
regulates the volume of the Accompaniment notes.  The pianolist is
theoretically looking at the wavering "Volume Line" that is printed
on original Duo-Art rolls.  This line is a visual aid allowing the
pianolist to know how hard to pump at any moment.  This pumping
determines the loudness of the Theme.  The foot-powered Half Duo-Art
uses a standard Duo-Art roll, but only the coding for the Accompaniment
accordion is being read.  It is up the pianolist to set how loud
the Theme will sound over the Accompaniment at any moment.

This is in contrast to a full electric Duo-Art, which has both the
Theme and Accompaniment levels set by coding on the roll that determines
how far the Theme accordion and the Accompaniment accordion are
collapsed at any moment.

Jason's last question is about what would the difference be between 
a Half Duo-Art playing a Duo-Art roll and a Themodist roll.  In the
first case, the pianolist is determining the volume of the Theme notes,
and the levels of the Accompaniment notes are set by the roll.  In the
case of playing a Themodist roll, the pianolist determines the levels
of both the Theme _and_ the Accompaniment.

One little fact that I like to point out to people is that originally
Aeolian made Pianolas that allowed the pianolist to express their own
personal interpretation of the music.  When Aeolian realized that the
public was beginning to demand motor powered, totally automatic players
that would reproduce the playing of master pianists, they built a
mechanism that would allow the owner to flip a switch that would allow
the Pianola to either re-enact the playing of a master pianist, or
allow the owners to interpret the music in their own way.  Thus, the
machine was called a "Duo-Art Pianola".

Randolph Herr