This is TiTAN 3X6mm Zero Play Titanium Screw (4).
New zero play titanium screw is designed for use on the upper deck and shock tower, the zero play screw with 3.00mm diameter on the upper side of the thread, it helps to remove the play and make the upper-deck and shock tower seat in the correct place without moving on the driving. there are 2 different specs, 6mm screw is designed to use on the upper deck and 8mm is using on the 3mm or 3.5mm shock tower.