23. The improvement of claim 22 wherein the receiver pair of upright arms is a first pair of upright arms, and the insert comprises a body and a second pair of upright arms, each insert arm having an outwardly extending wing, each wing extending through an aperture of the receiver, the insert groove being engageable by the lock and release tool for movement of the insert upward, downward, and rotationally.
23. The improvement of claim 22 wherein the receiver pair of upright arms is a first pair of upright arms, and the insert comprises a body and a second pair of upright arms, each insert arm having an outwardly extending wing, each wing extending through an aperture of the receiver, the insert groove being engageable by the lock and release tool for movement of the insert upward, downward, and rotationally. 24. The improvement of claim 23 wherein a pair of shallow grooves are formed in the outer surface of the receiver, each groove communicating with a respective receiver aperture, each groove extending from the respective aperture to a top surface of the receiver, each groove having a width for receiving a manipulation tool.