Sam Harris asked about felt like the Standard and Autopiano valves used.
You should try the non-stripped key-bushing felt. It comes in red,
like most of the original valve felts, and comes in 3 or 4 thicknesses.
I keep a couple of feet of this for various items in restoration: for
bushing wind motors, damper guides, soft rail brackets, etc.
I would try Schaff and American Supply for their NS bushing felts.
If you find the right key-bushing felts strips, then adding the NS to
the stock number is usually the stuff by the yard. I know I have some
of it in .050", but I do not remember where I got it. For super-thin
key-bushing felt try the nearest pool table repair shop. It is great
quality. Pool table felt is $80 a yard if you have it put on your
table, but if you go by and ask for all their scraps it is usually free.
D. L. Bullock Piano World St. Louis