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Conn used the model 15J for at least 2 different models of horn with which I am familiar. One was a 3 valve front action version of the 4 valve 5J, the other is a 3 valve top action convertable tuba. I would consider either of them to be 3/4 sized.

I think the early Conn 15J's were probably 4/4 horns. Later 15J's (have after 1988) were really King 1140's. I've seen 1140's with '15J' engraving on the bell. Were the King 1140's 4/4 horns? Not in my eyes!