The two tracker bar ports at the extreme bass and treble ends of the
Solo Carola tracker bar are to automatically operate the soft bass
and treble pneumatics connected to the bass and treble sections of the
regular hammer rail, same as on the Recordo system.  This enables theme
notes to be struck simultaneously at a level somewhere between theme
and accompaniment in either section or both.

Interestingly, the Solo Carola and Recordo systems were both developed
by the Cable Company of Chicago.  The dynamic information channels for
the Recordo system are blank on the Solo Carola tracker bar, making it
possible to create a unique player combining the features of both
systems.  I can't imagine this escaped the thought of Cable's engineering
department.  But apparently it was never done.

Incidentally, the Solo Carola rolls require twice the tracking precision
of regular piano rolls.  But, in spite of their pianos having no
automatic tracking, the rolls track perfectly.  Much precision was used
in making the spoolboxes and roll flanges, but the real secret lies in
the paper used.  It is the same paper found in Welte Purple Seal rolls,
and, usually with variations of color, nearly all "nickelodeon" and
band organ rolls.

Jeffrey R. Wood