I've asked about those numbers before and no one really seemed to have an answer then, either. I think Brilhart only made them for alto and tenor. Although Echomaster's reproductions include soprano as well.Never mind...I figured it out...it must be for tenor (standard size Ebolin/Tonalin). It threw me bc it came with a bari, on a Great Neck Ebolin bari mpc, but now I realize it isn't for that mpc. It looked like it fit pretty well at first, but once I put a reed on, I realized it could barely make it halfway down the body of the mouthpiece with the screws as loose as possible. Much better fit on tenor. They probably didn't even make them for bari.
I think you are right, at least for the Kenny Garret style ones. I think they might have made the more basic all metal one (not the B screw one) for bari, but those weren't stamped with anything. I've only ever heard of the numbers 3, 4, and 5 appearing on these. 3 is for standard alto, and I think 5 is for streamline alto. There doesn't seem to be much rhyme or reason to it. Part of me wonders if there is even a difference between the streamline tenor and standard alto. This is the third one of these I've come across in the last several weeks. The other two are already sold. Now I wish i'd taken some measurements of them. It would be interesting to know if the reed plates are different widths or not.I've asked about those numbers before and no one really seemed to have an answer then, either. I think Brilhart only made them for alto and tenor. Although Echomaster's reproductions include soprano as well.