TubeNet

My CB series fits that description exactly.

http://www.dougelliottmouthpieces.com/" target="_blank

I make 4 different rim sizes that are completely interchangeable on several cups. They are all available with bass trombone shanks, or tuba shanks in American, European, and Medium/Mirafone(between American and European).

The English receiver is actually slightly smaller than a bass trombone shank. I make those too.

I sell the CB series pieces mostly to Cimbasso owners and manly bass trombonists like Jeff Cortazzo in the Army Blues. But they are essentially small Eb or F tuba mouthpieces.

Bloke, that's what the Wick 4 and 5 are all about.

Aha! That explains the mpc shank size on an old Eb Besson my wife found. I took my Helleberg with me to test the horn, and it wouldn't fit. I was surprised because it should fit partly into a European receiver. Fortunately, there was a mouthpiece in the case.

Yes, indeed. The older British shank is .490 . My Besson BBb 3-valve comp has the smaller shank also, and functions perfectly with the Wick 1 mouthpiece, which was designed for Besson/B&H comps. The Wick no letter series are all the .490 shank, and are all standard catalog items always available. The 1 is designed for the BBb comps, the Wick 2 is modeled after, I believe, an old Kozy-Kup design, also for BBb, although those who use an eefer for the trenches also use them; the 3 is the "standard" eefer section mouthpiece, and the 4 and 5 are made on smaller blanks for smaller embouchures or smaller horns, as bloke has described in this thread.

I have 2 of these now (and used to have another). FWIW, if the Wick 4 and 5 are anything like my old Bach 25 then they are nothing like these tweener mouthpieces.

I have an unmarked 30mm and a Josef Klier KBP2C (shallow contrabone mp). The 2C is actually a bit shallower than the other. "X" has a shank that is somewhere between small and American- it will work in both my little King and a Miraphone (actually sounds pretty nice in the Mira). X doesn't work in my bass trombone because of the shank, but the 2C is awesome!!
Both work about equally in my King. My Yamaha 60B does too, actually. I think I'll try that one out more tomorrow.
I used to have a Kanstul 2A that was a heavier copy of the JK 2A, but I never used it that much in my old Conn Eb.

I am using the "X" all the time in my King, and it has a wonderful light/bright sound. I had planned to use such a mp when I was looking for small Eb for exactly the same reasons you wrote bloke.

The intonation on my horn is pretty good. When I have it tuned so that Bb and bass clef Eb are locked in, there are only 2 partials out. The 5th (but that's getting better), and unfortunately the 2nd. When I tune it so the 2nd partial is good, the 5th is fine, but everything else is just a bit high. I've been splitting the difference and pushing in for low Db and C and it's working OK, but not great. Everything else on this horn rocks. Sigh...

Yes, it is a little known quirk with King tubas that has been sorted out mostly and finally with 2341's of various eras, but you're right: King 2nd partials tend to be out of tune. I don't know why, either, but there is the connection between 2nd and 5th partials behavior on the various specimens of King tubas.