I have read with great interest varying opinions and ideas
regarding thin pneumatic covering materials.  I can speak solely
regarding singing bird automatons which use tiny two stage bellows
with one-way air valves.

In my opinion, modern alternatives fail miserably when compared
to zephyr skin or extremely thin, supple leather.  Both of these
materials can still be purchased today.  The modern alternatives
do not last (3 to 7 years maybe) and they are also much stiffer
impacting performance.

I have seen zephyr skin and thin leather covered bellows that I
know were from the early 1900s that still work perfectly -- many
of these bellows were hand dated on the covering.

On a side note, Reuge continues to use thin leather for their bellows
covering while another maker uses rubberized cloth.

Kind regards,
Troy Duncan
http://www.singingbirdbox.com/