Limited Edition TMS Kobo Original Sound Improvement Goods Rotary Spacer-kun (Brass Hammering Model) ① This is a sound improvement gadget to be inserted between the rotary caps of a horn. It changes the sound's responsiveness and resonance. The brass hammering provides excellent sound initiation. Can be used with Yamaha, Alexander, and Holton horns. I think it can also be used with tubas and trombones. Outer diameter 21.5, thickness 1.5, hole diameter 10.5