Abstract: A kit for building a retaining device with guide rails for the support of a contact carrier, engageable by a coacting plug-in unit, includes a pair of corner connectors molded integrally with the guide rails as part of an assembly whose elements are initially interconnected by a runner structure. The connectors, designed to engage the plug-in unit, are joined to the extremities of the associated guide rails by rupturable links facilitating their detachment. One connector is divided into a male and female part which are interconnected by a flexible link adapted to be looped about a handle of wire leads, emanating from the plug-in unit, when these two parts are snap-fitted together to engage that unit. Coupling formations on the runner structure enable the attachment of several identical assemblies to one another.