For me, the Selmer's shank diameter seems a comparable match to others. I have a Runyon 88, a Berg Larsen (London), and a Vandoren Optimum that all match up well. Likewise, I've tried a Ponzol and a few other modern pieces that all seemed a close match for the Selmers' fit. The only differences amongst pieces I've seen were from an old Buescher and Brilhart. The Brilhart was reasonably close to the others, but the Buescher was significantly smaller. There is no rule that defines shank size, but I imaging that a lot of modern pieces (not all) conform to the Selmer shank size on purpose.