Behind the fan plate there are 12 screws (not visible in the picture), and every screw has a nut with a small hook, that emerges through one of the 12 slots (visible in the picture). Each string is tied to one of these hooks. The 12 nice metal pieces you see above the fan plate are thumbscrews. On turning them, the nuts run along the slots, thus stretching the strings to pitch. Of course, there's a cavity in the guitar head that lodges the 12 screws and a small metal piece where the whole fan plate is anchored.