I used to get it from a plastics place, several around me sold it but I always ended up at a place on the old aerodrome on Purley Way.
I made carb spacers from it, I should remember the thickness as I have bits of it in my scrap cabinet, but can't.
We had similar guides at Longbridge on some of the spot welding fixtures, they used a hard wearing white plastic type material, Tufnol would burn/char and flake away with the heat.
Just flicking through a Tufnol manual here, and the varieties mentioned are :
Heron , Kite, Swan, Ketch, Carp, Lynx, Vole, Whale, Crow, Bear, Asp, and Adder.
Asp and Adder are asbestos paper based....
There are also Swan C.P, Grade 3P/46, and Grade 6F/45.
The feathery ones are paper based with phenolic resin apart from Crow, which is cotton fabric based with phenolic resin, along with all the mammalian types.
Not sure what a Ketch is, other than Jack Ketch, of whom the less said, the better.....