I would go one step further regarding Ed Sprankle's observations
on holes in the tracker bar:  my preference is for holes as well, and
as Ed correctly points out, if the system is kept clean and properly
adjusted it can be very responsive to wandering rolls.

There are two main types of adjustment mechanism, though: one moves
the upper spool and the roll to match the tracker bar, the other moves
the tracker bar to follow the roll.  I consider the latter system much
less likely to introduce roll edge damage.  Is that a fair view?

Roger Waring - The Pianola Workshop
Solihull UK