This listing is for a vintage "The Masteller" (Battle Creek, MICH.) baritone saxophone mouthpiece. I'm guessing that "The Masteller" was a music store/stencil brand made by another company. In fact, it appears identical to bari mouthpieces I've seen branded as Geo. M. Bundy and Conn. It is made of hard rubber, not plastic. It has flat inner side walls, and a medium round chamber. It is stamped "C", and the tip opening measures .080".
This mouthpiece is in good used condition. It does have an area of nicking on the table, but that doesn't seem to affect playability. The rest of the facing is smooth. There are no tooth indentations on the beak. The rubber is somewhat faded/discolored, and there is some ligature scratching on the exterior. This mouthpiece has been cleaned/sanitized.
Please feel free to message me with any questions.