A receiving member has a primary socket for a fixation mechanism and a secondary socket for a locking device. The secondary socket has a first bore intersecting a portion of the primary socket and forming an arcuate cut-out. The secondary socket has a chord member extending into the secondary socket first bore that forms a radial undercut in the secondary socket. The secondary socket has a detent extending from the radial undercut into the chord member. The locking device is rotatable in the secondary socket between an unlocked position in which an engagement surface of a cap of the locking device is angularly displaced from the arcuate cut-out, and a locked position in which the cap engagement surface occupies the arcuate cut-out and engages the fixation mechanism, and the arm engages an underside of the chord member, thereby preventing the fixation member from disengaging the receiving member primary socket.
