This is the locking knob originally supplied with the Arca-Swiss quick release, which applies pressure to the spacer sleeve which touches the vise grip jaw. It is very practical to carry an extra spacer sleeve and locking knob as insurance; the locking knob can fall off from prolonged vibrations, and the Arca-Swiss quick release is inoperable if either the locking knob or the spacer sleeve are lost or damaged from impact.
