Abstract: To provide security of attachment, between one container and another similar container adapted to be stacked on the top of the first container, each container has a keeper slot in one or more of its side walls adjacent the container floor and a spring leaf at the top of the container which may be bowed into either one of two stable positions. In one of these positions the leaf of one container registers with and locks within the keeper slot of a second similar container poised above the first container, and when the leaf is in the other of its stable positions it is disengaged from the keeper slot.