Rather puzzled by it, I took it to lunch, where I was peremptorily ridiculed for basically being too young to know what it was. As it turns out, this is the "type ball" of the IBM Selectric, an ingenious gadget that allowed one to change types on the fly (the ball had all the characters of a particular type on it, and would rotate as keys were pressed on the typewriter.
This story would be highly unremarkable if not for the fact that William Gibson, author of Neuromancer and cyberpunk pioneer, wrote a little rhapsody to the Selectric type ball.
[Dead Tech backgrounder, for extra points: In the decade or so prior to the advent of personal computers, IBM produced an electric typewriter called the Selectric; this had a “type-ball”, a metal sphere about the size of a golf ball, which held an entire font; you could switch fonts, which at the time was little short of miraculous; you could also, even more amazingly, power-correct mistakes with a built-in paper-colored ribbon. The IBM Selectric, when I started writing for publication, was the most shit-hot professional writing machine on the planet; by the time I could have afforded one, they were propping up broken barbecue grills in Value Village. The Finn’s shop probably has at least one box of Selectric type-balls, somewhere; they are beautiful sculptural objects, these balls, and won’t be easily thrown away.]