Abstract: Devices for production line assembling of upholstered seat back units upon upholstered seat base units include a pair of rigid coupling bars secured at their lower ends to the seat base frame and projecting upwardly from opposite sides of the seat base. Within the seat back unit, a pair of housings fixed to the seat back frame at opposite sides at the bottom of the back are formed with coupling bar receiving recesses into which the coupling bars are inserted by a downward movement of the back unit relative to the base unit. Within the housings, a locking member having a locking projection receivable in a complementary locking recess in the coupling bar may be driven by a wedge member slidable within the housing from an inoperative position accommodating movement of the coupling bar into the housing to a locking position wherein the bar is positively locked to the housing.