Hi Kent,
Not sure if this will be helpful, but...
A while back I restored a pair of Seventeens with the same grille cloth. One of my grilles also had a small stain that resembled rust. Like you, I also tried to clean it to no avail. Turns out the stain was isolated to just one horizontal thread of the fabric. Eventually that portion of the thread disintegrated into powder. I really don’t believe it was a conventional stain caused by a spill. Instead, I believe something in this weave is subject to oxidation and eventual disintegration. I ended up pulling a stray thread from the back of the grille and threading in to replace that missing portion. Not perfect, but worked pretty well.