The configuration of the model B trackerbar has been configured that
way to fine-hone it's compatibility to the Ampico roll coding format,
rather than to compensate for any deficiency of valves.

If one will notice, the only expression holes lengthened outside of
the normal hole line are #1 and #5 "crescendo" slot lengths in the
treble and the #3 sustain pedal hole in the bass.  These holes are
actually shorter (not longer) than the model A counterpart, as I
recall.  The #5 "jogged" hole in the bass end isn't an expression hole.
It times off the switch pneumatic.  It was jogged like that so the eye
could find it.  No other reason.

So the "few-thousandths of an inch" difference in the length of these
trackerbar holes between the two different model trackerbars are really
immaterial.  Do the math.

Craig Brougher