Headlamp / headlight panel screw, self tapping, Lucas. Used for headlights where threaded hole not used in shell on eg SSU700 series. Early headlighs MU and DU series have the 3BA screw. The later series with the thread insert have either 3BA OR 3/6 inch Whitworth and if no thread insert they used self tapping screws.
Original Lucas number:
186130 - screw