This is a BENGE trombone (euphonium) mouthpiece. I believe the model name is "MARCELLUS". There is a small dimple on the outer circumference of the rim, but it's small enough that it doesn't affect playing. I couldn't find the size when I looked it up, but it feels a bit wider than a Bach 6-1/2 when I play it. For reference, I've included images measured with calipers.