Note the one that looks like it on the right of the row of caps in the last link:
"mystery vintage gray cylinder (now identified- Mullard Mustard polyester film/foil),"
Which seems to confirm it as a polyester film capacitor (which as you can see was dipped in some kind of goo to seal it.) A film cap is the right kind to use for tone circuits. Stable values and low loss.
Generally in my tone circuits I use the Yellow ones a couple to the left which is essentially the same capacitor. I use them just because I have a drawer full of them.