It's a Wilkinson roller nut, which they used on the early Jeff Beck Strats. The current model has an LSR nut, I think, so presumably that's one of his old Strats with a big fat neck.
Beck's amazing and totally unique. But when I saw him a few years back, I must admit that by halfway through the show I was a bit tired of all the skronking noises and playing melodies with the whammy bar. I can only listen to him in small doses, after that I'm crying out for something simple!