| http://www.w3.org/ns/prov#value | - The assembly of claim 15, wherein the locking mechanism is a twist ring and a slider attached to the tubular body, the twist ring twistable to cause the slider to move between a first and second position, the first position temporarily locking the extension to the first poly-axial head, the second position allowing the extension to be disengaged from the first poly-axial head.
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