If there is a cloth backing behind the leather, on the inside of the bag bellows, then there is one repair that can work very well for years. That is by using screenprinting ink, like used to make images on T-shirts. I have done several very old bellows on medium format folder cameras that way, the oldest repaired four years ago and still blocking light effectively.
This would not work if there is only leather. The cloth backing holds the screenprinting ink in place. You would need a brush to apply it. Then curing is by setting the bellows in directly sunlight for many hours, until dry inside. After that you end up with a flexible repair that blocks light. Hopefully that could work for you.
Ciao!
Gordon Moat
A G Studio
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