Abstract: A coupling for making electrical connection of an avionics box and for retaining it in a rack has a rotatable jackscrew, by which cooperating parts of an electrical connector on the box and the rack can be drawn together. The jackscrew has a knob at one end with an indicator button which is pulled into the knob by a mechanical linkage when the two parts of the connector are fully mated. The linkage includes a sleeve which embraces the jackscrew and which is pushed towards the knob against a spring when the connector is mated. A ratchet mechansim limits the torque applied by the jackscrew. Ball catches engage asymmetric grooves on the sleeve to make it more difficult to unfasten the jackscrew than to fasten it.