Several years ago I was at a fabric covering forum (not the full workshop) at Oshkosh. I don't remember for sure, but I believe it was someone from Polyfiber doing the presentation, and he said there was only one mill making aircraft Dacron, and they were making it for both Ceconite and Polyfiber. They were both exactly the same stuff, except for the name printed down the edge. Of course, if you're doing a certificated aircraft you are required to use the chemicals that match what is printed on the cloth. And this is only a guess because I have never used Stewart, but I would guess it is also the same.
Dave