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I believe it to be true. Conn is not the only maker who made mouthpieces with non-metal rims a century ago. Conn also manufactured metal mouthpieces that had slanting rims for people born with unusual jaw formations.

I have never played one, nor am I aware of any being made for anything other than cornet.

In any event, time will have degraded the materials that were available at that time. I think, however, that they might sound very similar to the nylon rimmed or plastic mouthpieces available now.

I have seen a photo of one of these once. As you can imagine, the rubber from over a hundred years ago does not hold up very well, especially if played a lot. More common are the Conn "Visual Embouchure" mouthpieces with red plastic cups, but those were from long after Col. Conn passed on. For more information on the history of the Conn company you can visit Dr. Banks' website.

http://www.usd.edu/~mbanks/CONTENT.html" target="_blank" target="_blank

She's probably the most knowledgeable person on the history of Conn.