I've come across such tracker bars with suction grooves before, in
Welte and Hupfeld instruments, but never had a chance to investigate
them fully.  I'm pretty sure one of those companies held the original
patents, and Aeolian American probably picked up the design or maybe
just the tracker bars once they'd gone bust.

Ostensibly suction grooves allow a less acute angle for the paper over
the tracker bar and a shallower spool box, even when jumbo roils are
used, with less danger of spurious notes if the paper chatters.

However, Tempo governors prevent only increases in suction supply
being seen on the Tempo slide, and the paper speed for any given Tempo
setting is determined by the drag in the system, chiefly that of the
paper on the tracker bar and of the music roll brake.

The effective brake drag changes as the music roll diameter diminishes
significantly and the take-up spool diameter increases moderately; in
practice a judicious amount of braking force proves essential to make
the paper snugly spool and to reduce speeding up towards the end of a
reproducing roll.

Tracker bar paper drag is proportional to the suction levels on the
note holes and inversely proportional to the number of notes open.
Keeping the bar polished minimizes this drag.  I guess one idea of
suction grooves was that applying a consistent greater drag on the
paper during play would render fluctuations in note hole suction drag
comparatively negligible and keep tempos steadier.

Perhaps someone can tell us where the suction was fed from, and when.

Patrick Handscombe
Wivenhoe, Essex, UK