My records show that the pneumatic opening on these stacks is 1-1/4
inch, measured from the outside edges.  That is in agreement with
John Tuttle.

Another speculation about the reason for the graduation in size was
to compensate for the different weight of the hammers; the longest
pneumatics were in the bass and the shortest in the treble.  Aeolian
didn't always use this arrangement.  After a time they became one size.
They likely changed because they discovered that the smaller size
provided more than enough force to do the job and it was probably
cheaper to manufacture.

Howard Goldberg - Best Piano Service
Berkeley, California