I see lots of information regarding patching and repairing old bellows. Does anyone have any idea what these were originally made of? Definitely not leather, but an impregnated cloth?

"Keratol is one of the more popular prewar trade names for imitation leather. There were others. Believe it or not it's celluloid combined with linseed oil, applied to fabric, and a pattern then embossed with a roller. This stuff was developed in the latter half of the 1800's and used for suitcases, buggy tops, furniture, instrument cases, etc. In the 1950's it was gradually replaced by equivalent products made of vinyl on fabric."

"Fabrikoid was an imitation leather manufactured by DuPont.
Fabrikoid consists of cotton cloth coated with nitrocellulose. Among other uses it has been used for luggage, bookbinding, upholstery and dress trimmings. By the 1920s Fabrikoid was used heavily in both automobile seat covers and the tops of covertible automobiles.[1] Some of the early experienments on its uses in upholstery were conducted by Gilbert Rohde."