It looks like its linen to me and from the vintage it could be. Counted thread cross stitch on linen was very popular here around that time.
I think it will be a beautiful quilt top and its something that your daughters can look at and and know that their grandmother worked it and you constructed it!!
I'm going to look through my linen press now, I have some tablecloths worked by my grandmother and there could be a world of possibilities for them. Thanks for the suggestion and good luck with your project.