BOOBY TRAP - 1957

An absent-minded professor has developed a booby-trap device hidden inside a fountain pen. He demonstrates it to a group of soldiers at an army base. It is filled with high explosive and will detonate at the sound of Big Ben chiming on the hour, although only when the pen cap is removed. He later looses the pen inside a taxi and discovers that it was found by a nightclub barman who has since sold it to a pawnbroker.