Abstract: For the purpose of fastening instruments in an opening in a front panel or switchboard, connectors are provided which have tongue parts which are inserted from the rear into corresponding grooves along the side edges of the casing of each instrument. A cover plate projecting laterally beyond the casing abuts from the outside against the edge of the opening and the connectors press from the interior against the front panel or switchboard and pull the casing backwardly. The locking between the tongue parts and the grooves is effected by anchoring parts, preferably rows of teeth which are adapted to snap resiliently into one another and which act in the longitudinal direction.