Abstract: A piece of luggage or other container has a wall with a protruding structure which is adapted to be moved into a recess formed in a co-acting frame, with the protruding structure and frame having inter-engageable portions which extend along two sides of the protruding structure and recess and converge toward one another and have shoulders movable into an interfitting relation securing the container to the frame when the container is moved in a predetermined transverse direction generally perpendicular to the direction in which the protruding structure projects from the side wall of the container. The protruding structure is releasably retained against reverse transverse movement from the interfitting relation by appropriate holding structure, desirably taking the form of a key operated locking unit carried by the frame and engageable with the protruding structure of the container.