Prototype WTC stairwell sign. Interesting I would stumble on this a few hours before 9/11. When the World Trade Center was bombed on Feb 26, 1993 and we realized the hard way how unbelievably dark the stairwells become with no power, battery-powered lights were added (to supplement the main & generator-powered lights) as well as phosphorescent (glow in the dark) paint on the stairs and handrails and these phosphorescent signs on the inside of each door. Two prototype signs were made, this one and another for the non-re-entry floors (the stairwells could be exited only on every 4th floor with the doors locked on the other 3 floors). The other prototype listed the 2 re-entry floors above & below the current floor.