Yeah, there's probably some venders that sell this sort of thing, but if you know exactly what you need, then you don't have the scour the internet to find a guy that says he thinks he knows.
So, some really old Tubas have the same receiver as Besson medium shank. B&F stencil horns from Couesnon, Goumat, Bruno NY, etc... The kind of thing that you'd find in a catalog labeled "Contrabasse en Mib". This shank size is smaller than Bass Trombone shank, and larger than small tenor shank. It's also a different taper because of course it is.
Original specification is PROBABLY .460" (small end) @ 3 degree cone. I have had made a Josef Klier Tuba 9E on a 11.7mm shank with 1:19 taper. What's 1 MOA between friends? In the photo you can see that I damaged the Besson receiver a wee bit by using it to true up my original Eb Bass mouthpiece. lol... Other than that, it fits perfectly. If I run the mark through both horns, it is eventually completely removed.
Yes, it is possible to produce a Tuba mouthpiece in this shank with a backbore that plays in tune on Euphonium. My embouchure isn't really up to the task of playing the Euphonium with a toilet bowl, but I'm sure this is a very interesting prospect for many players.
Important detail:
Use a full length Tuba blank! Do not use the short shank style that the original mouthpieces and Euphonium mouthpieces have. It makes for a terrible backbore. Intonation is incredibly limited with the short shank! With the full length shank, intonation is acceptable up to 8th partial on a ~130 year old horn being played by a mediocre hobbyist. Working mouthpiece full length is 3 3/4" (~98mm).