Truck cargo box tonneau cover system including a side-locking lock bar locking multiple slats

A lock bar can be laterally slidable between an unlocked position in which the lock bar is laterally recessed relative to the side rail and a locked position in which the lock bar is laterally extended relative to the side rail. A side rail supported actuator can include a longitudinally extending actuator arm engageable with the lock bar to slide the lock bar between the unlocked position and the locked position and engageable with, and movable by, the tonneau cover as the tonneau cover moves from the open cover position to the closed cover position. Movement of the lock bar between the locked position and the unlocked position can simultaneously interengage and disengage, respectively, with each of the locking surfaces of the plurality of rectangular elements, which interengagement limits upward movement of the rectangular elements.