I'm pretty sure those "sheet" blankets were flannel. I still have a VERY old one that I slept under when I was a kid (60+years ago) and it is a heavy duty flannel. It is pink and coral and white large squares. No doubt it has shrunk as much as it's ever going to, so it would make a great batting for my next quilt but I'd have to be careful that the color of the sheet blanket doesn't show thru my quilt!