In numerous late style 2-tier Seeburg stacks that my shop has
restored since the 1960s, the original outside span was 1-1/8".
In the early style 4-tier Seeburg stack, I've seen original spans
ranging from 15/16" in some stacks to as much as 1-1/16" in others.

This depends on a number of factors: the brand of the piano used by
Seeburg (and in turn the geometry and functional dimensions of the
piano action), the length of the stack pneumatics (which varies,
depending on the piano), the thickness of the stop cushions inside
the pneumatics, the thickness of the pneumatic boards, etc.  Most of
these stacks are in pianos without a stop rail on the piano action,
so the travel of the pneumatic and pushrod is critical to the correct
operation of the action.

When restoring the stack for any piano without a stop rail, I restore
the piano and fine-regulate the action before deciding on pneumatic
span.  With the action regulated to work correctly from the keyboard,
I measure the travel of the wippen at the pushrod contact point and use
that information to help decide the correct span.  Pneumatic cloth will
last longer if the pneumatics neither hang open fully (which keeps the
cloth under tension when not playing) nor squeeze shut tightly (which
compresses the folds).

In a Seeburg stack, the felt punching between the guide rail and the
wood button on top of the pushrod prevents the pneumatic from hanging
all the way open and stretching the cloth, and the stop cushion inside
keeps it from tightly squeezing the folds.

If you haven't yet seen the Seeburg Registry at Mechanical Music Press,
click on this link:

  http://www.mechanicalmusicpress.com/registry/seeburg/seeburg.htm 

The Registry includes detailed descriptions and photos of Seeburg
pianos and their mechanisms, a dating chart, a timeline of mechanical
changes, and much more.  Click on the thumbnail pictures along the
right side of the page to see more pictures and descriptions.  The
buttons at the very end of the page lead to database reports that
describe over 900 Seeburg and Western Electric pianos that were
studied, providing the foundation of the research.  There is also an
online survey form if you'd like to add information to the database.

The left column of the page includes links to other brands of
instruments and to many other areas of ongoing automatic music
research.

Art Reblitz
http://www.reblitzrestorations.com/