This is a rare find, an M C Gregory “Master” 18M 5A Alto Sax Mouthpiece, from the late 1940s to mid 1950s, in Excellent Condition, with a ligature and Selmer Cap. This is the brand made famous by Paul Desmond, with the Dave Brubeck Quartet.
The 18M is a medium, round chamber with a 5 (.071) tip opening and an A (medium) facing.
If you are looking for that West Coast sound or want to add a different flavor to your mouthpiece collection, this is an ideal choice.
Please check the photos carefully, as I have tried to capture the mouthpiece from all angles.