Have never really bought the idea that all of what SMS material is original old stock going from the fact that it is often still off gassing when you received it at least in the past. Of course there were likely a number of manufacturers and they may not have found or someone had copied a supplier than was not one that supplied one or more of the plants. Very likely more than one was used for all plants - including those that did not build Mustangs
Not sure how many pictures I have of seats up close, normally taking pictures of stitching rather than the texture but will find some time to take a look