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I have stumbled onto a MP shank that I can't identify. It measures .536" OD at the big end, .466" OD at the small end, and is about 1.597" long. Right off, I can see that the taper is not Morse since it is only about .043" per inch instead of the required .050" per inch. I suspect this is European. A small tuba shank fits but is quite 'long'. A standard t-bone or baritone MP is too small. A bass 'bone 'AL' is too large. Any ideas?

It is most likely a european shank which is somewhere betwixt a small and large shank trombone mp. My willson 2900S euphonium uses this european size and switching mouthpieces is difficult because this shank size is fairly uncommon.