Cheap, effective, comfortable, this silicone thumb-rest cushion fits on every clarinet...even if you don't think it might. 

Want proof? Scroll through the images to see it on Selmer, Uebel, Royal, Buffet and Backun. And even though I don't have them handy, trust me: it'll fit on Yamaha, Bundy, Jupiter, Leblanc, and everything else. 

They work on A, Bb, C, D, and Eb clarinets. And probably F, G, H, I, J and K clarinets. 

Stock up, they're inexpensive and you know you'll need extra at some point!