This tracker bar is from a Cable Solo-Carola foot-pumped expression
piano.  Brass nipples were soldered onto the back side in the usual
manner; obviously the bar was heated to the point where they all
dropped off.  The journal, "Scientific American", published an article
on the Solo-Carola system which is reproduced in Harvey Roehl's
"Player Piano Treasury".

Roll perforations are of two different sizes; the small accompaniment
perforations pass between the holes in the upper row, while the large
theme perforations uncover holes in both rows.  Pairs of holes in the
upper row activate a set of 44 pneumatics in a stack located in front
of the tuning pins and forming a secondary hammer rail.

Since I have one of these instruments, additional information is
available.

Jeffrey R. Wood