RN Sea King Observers used them on overlays on the radar to plot ships, dip positions, sonobuoy positions, sonar contacts etc as the action plot. I remember it well. Very low tech but reasonably effective. In the tropics the pens were less effective as they tended to melt. Water based felt tip pens worked better but you had to buy them yourself as the Puss was notoriously tight arsed! Happy days!!