I believe the odd Cable tracker bar John Tuttle was asking about was
for the "Solo Carola" foot-pumped expression system.  It was basically
an over-complicated sort of Themodist setup.  The offset narrow slots
above the regular trackerbar holes were used to change the striking
position of the hammers, to increase the intensity of the notes.  In
Volume II of the Billings Rollography there is some information on the
system and the rolls.

I've seen a few rolls for the system listed on eBay in the past.  The
Solo Carola was a product of the Cable Company, released about the same
time as their Recordo.  The roll labels are similar to the early Cable
Recordo labels.

I've seen Solo Carola rolls from other manufacturers.  Rythmodik
produced some but there are others.  The roll hubs are extended on the
left flange about three-eighths of an inch.  The extended flange made
the roll chuck engage the Solo Carola system.

Rich Schommer
Alliance, Nebraska