Concerning style "O" orchestrion rolls, I would like to offer the
following:

In "O" roll instruments the top 24 notes of the piano scale are shared
also by the solo instrument or instruments.  When a solo instrument is
activated, the corresponding solo range notes of the piano do not have
to be cut off, but it is certainly an option since the piano is not
really necessary at that point.

Since "O" roll solo arrangements tend to be arranged for pipes and
repeating beaters on the xylophone or bells which sustain the notes,
the piano is not essential during solo passages since it does not have
this capability.

Some original orchestrions had a muffler felt on the piano in the
24-note solo section that would drop down in front of the hammers
during solo passages.  However, if you desire to split your pneumatic
stack into two sections, or make a pouch cut-off for the signal tubes
of the corresponding notes on your piano stack, you could turn off the
piano entirely during solos as in the Seeburg "H" orchestrion.  If I
were going to build an "O"  roll instrument that is how I would do it,
although the felt muffler would be easier to build.

"O" rolls should never be used on a machine that plays no solo
instrument.  The reason is that when the roll calls for the solo
instrument, the piano will drop down to a very weak melody line in
those 24 notes and the rest of the piano is just playing the
accompaniment.  The resulting sound will be severely lacking in the
melody.

When using the "O" roll with only solo instrument make sure that you
have it tubed to the correct solo instrument hole on the tracker bar
that makes it come on whenever a solo arrangement is evident or you
will have the same resulting loss of melody.

Some original tracker bar specifications label the extra instrument
holes as "xylophone or bells on" and "violin and flute pipes on".  In
building a new machine it doesn't matter which instrument you use on
which set of holes, as long as there is always a solo instrument on
during solo passages.

Reiterating beaters should always be used on the xylophone and bells
in "O" roll machines but I personally prefer single stroke on bells.
Additionally, wood beaters on the bells will be less reminiscent of a
room full of telephones ringing.

Passages requiring the solo instrument are easy to see on "O" rolls as
well as to hear because during solos the notes shared by the piano drop
down to only one or two at a time.  When no solo instrument is on the
piano plays full arrangements.

The reason "O" roll solo arrangements consist of such few notes is
primarily due to the fact that original Coinola orchestrions which used
"O" rolls had very small pressure pumps, and would have been unable to
have enough wind reserve to blow many pipes at a time.

I invite any comments or corrections as always.

John D. Rutoskey
Automatic Music Machines
Baltimore, Maryland