I have a Chickering upright Stoddard Ampico with the loud/soft control
mentioned, and mine works.  I'll have to look at my mechanism more
closely, but as I recall there are 1 or 2 small pneumatics each on the
bass & treble expression boxes which push the expression throttle rod
(sorry, not a very technical term!) up & down to override the paper
roll control of loud & soft.  I'll try to give you better details in a
few days.

-Rob DeLand¶
(deland_robert@macmail1.csg.mot.com)