Some years ago I rebuilt an Aeolian style 1500 player reed organ
which has exactly the same small pneumatics you show in your pictures.
The Style 1500 is a 46-note organ which operates on suction.  I found
a date penciled inside which would indicate that it was built in 1895.

I used thin pouch leather to cover those pneumatics.  The suction
does pull those flat when you are playing the organ.  I measured the
suction once and I believe the organ operates on about 2 inches water
gage.

If your organ also has the long thin pneumatics which push a rod
which opens the pallet valves inside the wind chest, be sure to use
a thin flexible leather on that.  The original leather looked like
what Leather Supply House calls chrome pneumatic hairsheep.  These
pneumatics have so little volume they don't have the power to overcome
much resistance.

Howard Wyman
Valrico, Florida