Could this be Pellon interfacing which you use in garment construction? It comes in several weights and is fusible or sew in. I can see how that would help with redwork because sometimes it is hard to make sure your threads don't shadow through. I'm going to try a little piece with the fusible because I can see how this could really work.
I just love hand embroidery, esp redwork. I inherited several sets of embroidered pillowcases from my mom. I didn't realize until later that her mother had made them. They were beautifully embroidered with handmade lace on the edges. I was shocked when I learned that my grandma had made them. My memories of her are of a large very unfeminine woman with a cigarette. I guess she did gorgeous handwork of all kinds. Since some of the pillowcases were worn out, but the edges were good I took them apart and made aprons using one for the bib and one as a border on the skirt. They are darling. Sorry for the trip down a rabbit trail. Lots of good memories here.