My grandmother told me that people the generation prior to her used walnut hulls to dye fabric. She was born in 1905. She said that it was a common pioneer thing. If my memory serves me correctly, she also had a quilt back dyed in this fashion. It seemed to be from the 1850-1890 era. I do suppose, though, that people could have done it later if they knew how to.
What a wonderful item you have on your hands. I do hope that you can solve your puzzle. Sorry that I don’t know anything about the Dunlop.