I've been playing my trusty 321S since 1979 and have tried just about every "common" small shank mouthpiece but I always end up coming back to my Giardinelli 4D. It gives me a certain "wabi-sabi" factor that I really like. I believe it is 25.4mm, similar to a Bach 6 1/2 AL. Unfortunately, they're no longer being made but you might see one occasionally on EBAY. My second choice is the Alliance 6S and my third choice would be the Bach 5GS. As you've probably read, mouthpieces are a highly subjective and personal choice. I caution you however, that trying different mouthpieces is fun and somewhat addictive. Be careful that you don't end up with a box full of mouthpieces in your closet that you never use...like some of us! ;-)
Last edited by FedXav8r; 03-27-2017 at 10:19 AM.
1979 Yamaha 321S Euphonium
1965 Conn 7G/6H Valve Trombone
2014 Wessex BR140 Baritone