One of the most elusive items to source, the Ground Zero Indicator (GZI). A simplistic pinhole camera. When a Nuclear Bomb explodes the flash of light goes through the pin hole and burns a mark onto the negative graph paper inside. This allows you to calculate high and direction. It is the rarest of the items next to the Bomb Power Indicator and FSM Pipe. The second picture solved a mystery. What was the big protractor for? It was used on top of the GZI for use in Metar reporting. Most posts didn’t have this item so they marked the 360 degrees on the top with marker pen (which was a lot easier than clunking the protractor about).