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...what you are describing is a Cinelli 1R stem that has a small wedge that expands against the bar from the back. In co-op settings, this small wedge is often lost, because it's not fastened to the rest of the stem in any way, and often the person who removed the stem and the bar from it loses the little wedge thingie.

They are pretty much useless without it. I just put one on a Raleigh Team Pro yesterday, and had to borrow the wedge fronm a different stem I probably won't use.

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