With that in mind, I find that these Schilke tuba mouthpiece shanks and old Conn Precision shanks meet Matt Walter's .530 inch European gauge, and then some.
The Conn Precision line's most familiar model is the Conn 2, and in particular without any hint of a shoulder at the top of the shank, just a smooth, unmarked transition from the shank to the body flare. (The shoulder appears in the later Improved Precision line, according to the Conn Loyalist page.) I measure my Conn 3 even larger than the Conn 1.Matt Walters wrote: EUROPEAN SHANK: This is an increasingly popular shank size of about .530" diameter at the small end of the shank. It is found in mouthpieces like the JK Exclusive, Perantucci, Laskey "E" shank, and of course our wonderful Sheridan Series of mouthpieces. Most every European and American tuba built these days, will accept this shank size.
I'd be interested to see a report, from someone with more reliable measuring abilities and a nice round-ended shank of one of the above, or for that matter any old US made mouthpieces that might turn out to have "European" gauge shanks.