The module has opposing guide rails (3), used simultaneously for insertion of a circuit board (1) relative to the module (2) and for insertion of the latter relative to the appts housing. The guide rails have spaced split projections (4) along their length, extending inwards for securing the sides of the circuit board. The outer surfaces of the guiderails are profiled to cooperate with reception rails attached to the side walls of the appts housing. Pref. the rails have a nose at one or both ends fitting into a retaining opening in the sidewall of the module.
