Abstract: A removable frame is detachably connected to a supporting frame by a connection having a plurality of spaced apart vertically extending sleeve-like members mounted on the removable frame. Upwardly projecting members are mounted at the periphery of the supporting frame and telescope into each of the sleeve-like members. A horizontal tubular housing attached to each sleeve-like member carries a locking element which cooperates with its upwardly projecting member to form a positively locked connection between the removable and supporting frame.