A20-A21 Neoprene Bumpers
Bumpers are made of neoprene, preferred by some players, because it bounces the valve back faster than cork.
A20 Neoprene Bumpers for French Horn
A21 Yamaha Neoprene Bumpers for Tuba (.175) Diameter: .25
(Can be found on page 140 in the yellow 2008 Ferree's Tools Catalog)