Hello John and MMD,  The tracker bar is from a Cable Solo-Carola player
piano, and is later than 1906.  The Solo Carola had an additional set
of pneumatics that moved the hammers (in groups of two or three) closer
to the strings in proportion to a signal derived from the long slotted
holes above each playing note.  When playing Solo Carola rolls, the
piano could provide individual note intensity control.  I believe it
is pictured and described in either the "Player Piano Treasury" or the
"Encyclopedia of Mechanical Instruments".  I have never heard one play,
so I cannot attest to its ultimate capability.

Bob Hunt