I've never knowingly (is that a proper word?!) seen feedsack. They all felt like cotton although some were a little stiffer than others. Considering that a few of the hexagons were scraps of the same fabric pieced together to be big enough for the 2" hexagon, I would imagine they collected usable fabric from whatever source was available. Unfortunately, I wasn't interested in quilting until several years after my mother died so I was unable to ask her about the fabric.
Gramof6, that green is one of the reasons I decided to applique the blocks on an antique white background. I could not match the green. And this was my mother's first quilt work and many of the outside hexagons were misshapen making attaching new hexagons almost impossible. There was some very 'creative' ironing necessary to get them in shape to use.