Abstract: A lock rod support member can be stowed within a support tube housed in the body of the pick-up truck on which a supplemental tailgate assembly to be deployable from the tailgate structure of the pick-up truck. The support tube for the lock rod can be mounted internally of the D-pillar of the pick-up truck body with the lock rod extendible therefrom, or mounted beneath the top rail of the pick-up truck bed from which the lock rod can be extended, for engagement with the deployed supplemental tailgate assembly. A stop bracket positioned at the end of the lock rod support tube engages and positions the formed distal end of the lock rod for appropriate positioning for engagement with the supplemental tailgate assembly. A spring latch mechanism is mounted on the opposing ends of the supplemental tailgate structure for clasping the insertion end of the lock rod members.