Abstract: A package comprises a cover closing an article housing formed in a baseplate. A contiguous compartment closed by a plate contains a spring assembly comprising two arc members joined at their ends and bearing on abutment members. A profiled rod inserted through a complementarily profiled opening pushes on thrust member against the abutment of the ends of the spring assembly on the abutment members so as to bring about, by symmetrical reaction, retraction of the bar supporting the bolt members. However, if a fly-weight has not previously been retracted by a magnetic field a projection engages in a groove in the fly-weight to lock the lock. The consecutive action of the magnetic field and the pressure on thrust member are applied by a key device retained at the checkout of the self-service store. The invention has one particular application in the merchandizing of prerecorded magnetic tape cassettes.