There is an excellent article in the MMD Archives written by Craig
Brougher explaining the principle behind which the graduated pneumatic
system used in Duo-Arts work.  It is dated 1997.02.27 and entitled
"Duo Art Graduated Pneumatics".  It was basically to compensate for the
varying mass and velocity of the hammers under uniform vacuum supply.
You may note some of the pneumatics in your stack also have 'assist
springs' installed in the hinge ends of them.

One thing I'll add is that in these graduated stacks I've worked on
that were in totally untouched original condition, some of these
springs were adjusted (bent) or completely pulled loose on one leg.
This would seem to have been done at the factory using a long tool
inserted between the pneumatics in order to 'fine-tune' the
compensating effect, much to the same effect as the later 'note
compensation' used in the higher-end versions of the Model B Ampico
stack.

John M. Runge