This is a plastic button that sits where the left thumb rests on a saxophone. I believe it's called a thumb rest button. Here's an approximate measurement of the body: the recessed part on the back is 16.6mm, the height is about 8.7mm at the highest point and 6.6mm at the thinnest part, and the outer diameter is 24.4mm. There may be some manufacturing variations. The last photo is not of this product, but is included as an example.