I have to agree, if it aint broke don't fix it.
The heritage series mouthpieces are the same as the traditional wicks in cup size/shape, backbore etc. They just changed the OUTSIDE design.
Generally, the letter following the number usually refers to the Cup Shape/depth, followed by a second letter that refers to the backbore/shank size. But different letters mean different things for different manufacturers. Visit the link below for comparisons between the major brands.
http://www.markjacobsmusic.com/Mpiece.htm
Before you decide on a mouthpiece, you need to know what you want to sound like.
Do you want a bright, edgy sound?
Do you want a powerful, commanding sound?
Do want a warm, colorful, sound?
Do you want a dark, heavy sound?
I suggest you listen to trombonists and determine what you like and then find a mouthpiece that gives you that sound. DON'T be concerned about all the microscopic details and measurements, just the basic understaning of how each mpc compares to another should help you. Don't be drawn in by what the manufacturer says a mouthpiece will do for you. You have to play on it!
I prefer a warm, colorful soung, but something that can be powerful and can cut when needed.
I have played on Schilke 51D, Schilke 52D, Denis Wick 4AL, Denis Wick Steven Mead 4AM and 5AL.
I have tried so many mpcs, and have spent over $1000 in the past 7 years.
Now...I play on a generic KING 6 1/2AL on my King 4B Sonorous Trombone, and a Brian Bowman BB1EU on my Willson 2900BS Euphonium. And for Bass Trombone I have used a Schike 52D and a Bach 4G. NOT standard bassbone mouthpieces at all and generally make me play with the tuning slide about 1 to 1 1/2 inches out.
Any way I have said enough....I hope this helps.
Raul E.