In response to Tom DeLay's query, the knob in question is "Soft-Loud."

I have a 1920's vintage "L", and the tubing from the on/off control
goes to a valve unit controlling stack vacuum.  When the knob is in the
"Loud" position, an assigned perforation on the "A" roll automatically
lowers the stack vacuum, when that perforation is open.  When the knob
is turned to "Soft" the stack vacuum is lowered permanently during
playing.  This is achieved by a regulating pneumatic that is fastened
to the front of the distributor unit, located on the bottom left side
of the piano.

David Oppenheim