Yes, the European shank is intermediate in size between the large and the small shank. I strongly recommend that you do NOT attempt to replace the receiver with a medium one (unless that was the original one on it) since the range of mouthpieces currently available in medium shank is drastically restricted compared to those available in small and large shank. In fact, one reason (see my sig below) that I have the Doug Elliott mouthpieces for my two euphoniums is that replacing the large shank with the medium one allows me to use the same mouthpiece (bowl and rim) in both my Mack Brass compensating horn and my Amati oval euphonium. Otherwise, I found that the best I could do for the medium shank horn was one of the SM mouthpieces (which I don't particularly like), and the largest I could get was a size 4 (and I wanted something a bit bigger).
It may well be that a previous owner -- being discouraged about finding medium shank mouthpieces -- had an adapter fitted, and it's a bit small for a small shank mouthpiece.
If you think there's an adapter in there and it's stuck, you could initially try to get it out. Squirt some PB Blaster into any seams you see between the adapter and the receiver itself. Try to orient the horn so that the solvent will dribble down between receiver and adapter (so have the receiver pointing up). Repeat this process on a daily basis for, say, a week. It sometimes takes this amount of time for the PB Blaster to works its way into the correct places; but it's amazing stuff. At that point, see if you can (CAREFULLY) grab the end of the adapter with something like padded channel-lock pliers and start to (GENTLY) turn it. Applying torque in this way will often break apart a slide or adapter where simple pulling won't. You want to be VERY careful that you don't twist your mouthpipe or receiver.
Or take it to a repair tech and he'll do all of that for you, and maybe apply a torch to it if that's necessary.
Last edited by ghmerrill; 08-21-2013 at 09:56 AM.
Wessex EEb Bass tuba (Schilke 66, Denis Wick 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE N106, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba, Denis Wick 5