In response to the Cable tracker bar question, the tracker bar
that John Tuttle is inquiring about is a Solo Carola.  It was a very
feeble attempt to add expression to the rolls.  There were tabs that
held the hammers forward and when a note was to be accented, the
hammer would fall back.

I have seen only one unit.  Most of them were removed by impatient
tuners who didn't want to be bothered with removing the unit in order
to tune.  All in all, I think it was a bad idea.

Don Barton - Barton Player Piano Co.