Abstract: A load bearing platform mounted in a vehicle box for reciprocatory rearward load discharging and forward return movement between the front and rear ends of the box, and a transverse load engaging wall structure mounted in the box above the platform for material unloading movement from the front end toward the rear end of the box. A pawl and ratchet connection between the platform and transverse wall structure imparts step-by-step load discharging movement to the transverse wall structure responsive to reciprocatory movement of the platform. Latch mechanism releasably holds the transverse wall structure at each step of its movement during forward return movements of the platform. A pair of side wall elements or wings project rearwardly from opposite ends of the transverse wall structure closely adjacent the side walls of the vehicle box to aid in reducing lateral pressure of the material against the side walls of the box during discharging movement of the platform and transverse wall structure.