35 Terraplane Upholstery Question

35 Terraplane

Hello All,
During my wiring project I'd seen a few walnut shell pieces in the trunk and decide to removed the back seat to investigate.  The back rest of the seat was about a third full of old dried empty walnut shells.  In cleaning them out I found what remained of the previous tenant.  There are walnut trees around my father in laws barn were the car spent about 26 years.  Anyway, I removed the backing board on the seat.  I've attached pictures of the material that was between the backing board and the wooden seat frame.
Question, does anyone know what this material is and were and if it's available?
Thanks for the help
Tom