This is a close-up of the bottom right corner of the UFO image in the opening credits of the X Files, the first image after the show title card.
The text says:
FBI PHOTO INTERPRETATION
EX69357012/55328 WASH HQ
The first two lines are pretty clear, especially in the context of the show, but does anyone have any clues about the third line? All my googling has turned up squat so far, not even a lead to follow.