A base part has a planar inner face, and a cover part is shiftable on the inner face between a closed position overlying and congruent with the base part and an open position exposing at least part of the inner face. A pair of parallel elongated spring elements extend parallel to the inner face in one of the parts. A pivot in the one part between the center portions carries a nonround cam for movement between a pair of end positions engaging and slightly transversely elastically deflecting the center portions and a through a center position engaging and greatly transversely elastically deflecting the center portions. The cam is connected to the other of the parts to pivot between its end positions and through the center position on displacement of the cover part between the closed position and the open position.