[ In MMDigest 001109, "Unusual Tracker Bar in Weber 88-note Player",
 [ Paul Camps wrote:

> In the centre of the tracker bar there are 16 pairs of vertical
> slots positioned above the note holes: the first pair are above
> note 30, and then above every other hole 'till note 60. ...
> My guess is that it would have detected the alignment of the
> perforations in a music roll, providing that something was playing
> in that part of the roll.

This system was known in the UK as the Philipps tracking system,
thanks to its earlier appearance on Philipps Ducanola foot-pumped
and Duca reproducing pianos.  I don't think it originated there,
though.  Very roughly, it manifested itself in these makes at these
periods:

  Philipps 1906-1923
  Aeolian 1924-1926
  Angelus 1926-1931

The system worked as you imagine, just like two-hole tracking using
the edges of the roll, but using central notes on the roll.  In the
pianos I've seen with working systems the tracking is performed
using non-spring-loaded double pneumatics so that the roll can go on
some way without notes in the central region, by simply staying in
the position last sensed.

This made conversion to orthodox double-finger tracking especially
easy -- rebuilders long before the 1940s were doing this because the
slots were even worse than the Standard four-hole system for filling
with paper lint.  In Aeolian makes of instrument (which included some
Duo-Art instruments) it was an unmitigated disaster, for this
reason.  There is evidence that Phillips slotted tracker-bars went on
being used on new instruments with double-finger or four-hole edge
tracking added to them for some years.

I haven't seen enough Duca or Ducanolas to say whether they did
something more sensible, like provide unscrewable lint filters.
But their rolls were of higher paper quality, so maybe that had
something to do with the long period Phillips used the system.

I have seen some recent restorations which use the system, one of
them an Aeolian instrument where the tracker-bar had Philipps slots
and two-finger tracking had been added, now to be removed again !
If it's carefully maintained and kept clean, it works very well.

I think if I'd been a player maker using this system, I'd've provided
a couple of rubber bulbs for the owner to puff the lint out every so
often.  They would have acted as suction reservoirs to damp the
tracking action, too.

Dan Wilson, London