It's a variation on a proprietary pickup mounting system that allows you to securely mount a pickup directly to the body of a guitar (the original used a bell-brass block, I don't know what FW's using). The block is mainly there to provide a place to mount the four machine screws that sit under the pickup allowing the height and angle of the pickup to be set, before firmly screwing the two normal (what would be height-adjusting) machine screws into the base-plate to lock the pickup securely onto the guitar body, so that the pickup resonates with the body, and doesn't vibrate slightly on the usual pickup-ring and spring arrangement.