We had one in our high school darkroom, which was also used by a vocational program in offset printing. There was also a big copy camera with the shooting stage in its own room, and a vacuum back that opened into a space adjacent to the darkroom.
I remember the way it worked was you would put your negs emulsion side up on the glass (the bulbs, individually switchable, were underneath), paper face down on the negs, and then you would lower the back. When the lock clicked in, the bag inflated, and when it was full, the lights came on for the exposure and turned off at the end of a preset time. Click--bzhzhzhzhzhzh--zap.