Spacer sleeve for early-model clamps.
If you lose the knob that's on the quick-release clamp (applies only to non-captive early-model screw-knob style clamps), you'll probably lose the tubular spacer, too. Order a replacement spacer for $6.
For a quick field fix, store a spare spacer and a metric-6 stainless-steel wingnut. Winguts can be ordered at $2 each.
NOTE: the internal diameter of the spacer tube is different from one end to the other. When placing the spacer onto the shaft of your clamp, be sure to mount it with the LARGER inside diameter end on first.