There are two types, ones which have the unthreaded shoulder, about 12mm wide, and ones with no appreciable shoulder at all. I have not seen any in between. And as mentioned in that post, make sure you ge the right ones, which on the 2a should be the ones with the unthreaded shoulder. If the clamping section of the tube has no thread, it must clamp on an unthreaded shoulder.
John


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