Cygnus- I would bet that your mouthpiece is hard rubber. Too bad some Martin guy didn't see this yet, but I would guess the tip opening on one of these standard (included with the horn) pieces is around .065 to .075. You can check at mouthpieceheaven.com for info about tip openings and chamber sizes and such. But they have no specific information about Martin mouthpieces. I would suggest trying some mouthpieces at a large music store if possible - using a tooth guard of course - before buying. Also try some vintage hard rubber pieces if possible. A Selmer Soloist scroll shank would sound good on your horn - if it's an old Super Balanced Action. You seem to have 1 too many digits for that particular horn. Finding more than a D opening might be tough though. I use a Soloist F (around a .090 tip) quite a bit with my old Selmer Tenor. In terms of new mouthpieces, Patrick Springer makes an excellent hand finished piece for a competitive price, and he will recommend sizing and proper model for you through his website at www.springermpc.com - - Good Luck.