Abstract: A The invention discloses an assembly consisting of a head (1, 1′, 200) and an overcap (30, 30′, 30″, 300), the head having a cylindrical neck provided with a helical screw thread (21), the overcap having a cylindrical skirt (31) provided with a thread (36) matching the neck thread (21). The overcap carries an axial plate (33) radially flexible and circumferentially rigid. The neck base (10) is provided with at least an opening stop (12), and at least a closure stop (11), of lesser height. When the overcap (30) is screwed on the head for mounting the assembly, the free end of the axial plate is urged into contact with a first wall (14) of the opening stop (12) and the axial plate whereof the free end is urged by a centrifugal motion, is elastically deformed then, folds down towards the neck in a space between the opening (12) and closure (11) stops, its displacement being thereby restricted.