They're a waste of paper these days, but back in 1977 the Yellow Pages were an essential communications tool. And in order to more easily produce ads so the thick tomes remained free, Bell Labs (now AT&T) developed a remarkably advanced desktop publishing system.
Compared to the graphics software like Photoshop and InDesign we use today, the system looks incredibly primitive. But you have to remember this was 1977, and being able to quickly digitise artwork with a scanner, or even adjust the size of text using a stylus and tablet, was revolutionary. If you had spent a career slicing and gluing bits of paper together making ad mock-ups, this would have blown your mind.