I don't know what the "proper" historically accurate glue is for gluing
leather to rubberized cloth.  I am glad you asked and hope there are
good answers to your question.  I do know, that if I use PVC-E or
contact cement, it works really well for this purpose.  Also, it will
not create any havoc for future restorers -- no need for solvents and
scrapers as long as these proven, strong, yet newer flexible adhesives
do not touch the wood.

I find that hide glue and fish glues, that I'm aware of, seem too stiff
for above the hinge of a pneumatic.  I strongly believe that an adhesive
used should not encumber a pneumatic from opening or closing.  There
are always going to be those out there who condemn my opinion, but
I use contact cement where the cloth overlaps as opposed to hide glue.

If there is an equally flexible hide glue that adheres well and I find
out about it, I will use it instead.  I will clean the bellows cloth
with solvents and "scuff it up" whatever I have to do to get this
flexible hide glue to adhere.  I am obviously not a chemist or scientist,
nor are the vast majority of player rebuilders.  We are often "flying
by the seat of our pants" doing the best we can in many instances.

Bill Maguire