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Sounds like pre-1980's instruments which had smaller mouthpiece receivers.

Denis Wick mouthpieces to fit these smaller receivers are still available. Model numbers 2 and 3 are probably the most widely used.

This was just discussed here:

viewtopic.php?t=10129

Your best bets are the Denis Wick mouthpieces without the "L" designation. They have the smaller, European shanks. They aren't very expensive either. I would recommmend a 2, 3 or 4 for middle school students on BBb instruments. I like the 2 for BBb and the 3 for Eb, but the 2 might be a little too large for the average middle schooler.