This is a solid round rubber material for use as rotary valve stops. It's quiet and long lasting and is available in several diameters to fit most any size brass instrument with rotary valves. It is easily trimmed with a small knife. We sell it by the inch, but if you order more than 1 inch of a particular size, we'll usually send one long piece cut to the length of your desired order.
Shires trombones use these sizes:
Axial: .27” (17/64” is close), or 6.86mm
Rotor: .209” (7/32 is a little large, but closest fraction), or 5.31mm
Tru-Bore: .3085 (5/16” is a touch large), or 7.83mm
Fractional larger sizes can be trimmed to fit.