Hi all,  Let me start by saying I have heard only four or five Hupfeld
Phonoliszt Violinas (HPV) in my travels and I have never worked or
studied one.

A violin bridge mute quiets a violin and can be used during practice
so as not to disturb others.  It would also sweeten the music, i.e.,
alter the harmonic content like the una corda pedal (key bed shift)
in a piano.  I think some can be adjusted to different mute levels.
I have not made note of, looked hard at, or examined a HPV mute.

If I owned such an instrument I would treat the mute adjustment like
a soft petal or key bed shift adjustment -- adjust it so the violin can
still be heard, i.e., not too overpowered by the piano.  The adjustment
should probably be how hard the mute touches the bridge or how compliant
is the support between the activating pneumatic and the mute, not how
far it is from the bridge when turned off.

The key bed shift and sostenuto pedals are not used as frequently in
normal piano playing as the hammer rail lift is in the reproducing
piano recordings, but they serve a purpose in piano playing just as
does the hammer rail lift in the reproducing piano.

By the way, my guess is that the hammer rail lift was used in order
to have the grand and upright pianos sound as close to the same as
possible when performing in the show room.

Most, if not almost all, uprights to my knowledge did not have a una
corda or sostenuto functioning.  I have heard in the past that Charles
Stieff did make an upright piano with a functioning sostenuto petal;
I cannot say I have seen one but several members of the Baltimore
chapter of the Piano Technicians Guild have said they exist.

Allen Ford

 [ I once played a large upright piano by Heintzman & Co., Toronto,
 [ with a full sostenuto mechanism that functioned very well.
 [
 [ In a reproducing piano, the purpose of the grand piano hammer rail
 [ lift system is simply to reduce the hammer velocity for a given
 [ force applied to the key or wippen, same as in the upright piano.
 [
 [ -- Robbie