I just recently bought a used Antoine Courtois 155 and like the way the horn plays, but I'm having a hard time figuring out if its got a special shank or I'm just being too picky. It came with a mouthpiece that looks to be labelled as a Antoine Courtois Paris 11GP or 11SP? It's hard to tell with the stamping. The trombone seems to measure pretty close to a .547 bore. I have a large shank Bach 6 1/2 AL that fits in it mostly except it rattles a bit back and forth. I have a Stork Custom 1 that rattles the same amount back and forth when seated. The Courtois mouthpiece does not have that rattle. The Courtois mp is a bit small for my taste (since I primarily play bass on the Stork) and I'd like to try other mouthpieces but I wonder if there is something special about this older trombone I should know before I go looking at mouthpieces? I can't find any info on the trombone or mouthpiece and am wondering if the rattling is a sign its non-standard for large bore?

I ended up talking with Doug Elliot and he got me sorted. Just wanted to follow up with another recommendation for Doug!

I am curious what information Doug shared with you to resolve this? Does the instrument have a differently tapered mouthpiece receiver?

Yes the receiver had an improper taper of some sort. I sent him a scrap shank and lapping compound to lap it to something closer to the correct taper.

Probably a safer method than reaming.