Hi All,  I have a slightly different take with regards to why Aeolian
used tan pneumatic leather between the deck and the striker pneumatics.

As Standard and other manufacturers discovered, the force presented
to the piano action by solid (or hard) linkage was relatively severe or
abrasive, so 'flexible fingers' were introduced.  Other manufacturers
accommodated the problem by increasing the thickness of the felt
padding between the strikers and the wippens.  Regardless of which
technique was used, it seems somewhat obvious that the 'snappiness'
of the pneumatic in conjunction with hard linkage was not desirable.
(Evidence other techniques like thick felt punchings, relatively thin
striker fingers, and 'soft' hinges.)

Those who have rebuilt the Duo-Art stack are aware that pneumatics,
which are 100% air-tight before being mounted, are not 100% air tight
after they are mounted because of the very slight amount of leakage
presented by the leather.  Also, the pneumatics are not 'stiff': they
have a slight amount of 'give', also because of the leather.  Together,
these two factors help reduce the 'snappiness' of the pneumatic, giving
it a more human touch without reducing the repetitive capability of the
pneumatic.

As others have conjectured, I too do not believe that Aeolian had and
eye on the future with regards to the usage of tan pneumatic leather
between the pneumatics and the tiers.  I strongly believe it was done
purely for aesthetic reasons: to improve the human nature of the
mechanism.

But don't get me wrong -- it pleases me to no end that Aeolian used
the leather, and I always replace it when I restore a Duo-Art stack.
Replicate the original to replicate original performance.

Musically,
John A Tuttle
Player-Care.com
Brick, New Jersey, USA