The Pinarello Gan seattube features an integrated seatpost fastening system that employs a wedge that fits in the frame and is tightened via two threaded internal hex bolts that push the wedge against the post. The system maintains the aerodynamic design and shape of the seattube/seatpost junction and actually secures the post very effectively. It is not without challenges, as every time you remove the post, you need to take care not to lose or misplace the wedge or bolts. If you do manage to lose track, Pinarello offers this top quality OEM replacement set.
The wedge itself is machined precisely from aluminum and has a lightly textured surface that provides friction to the back of the Gan aero post to bind it in place once the so-called "grub screws" are fastened. Please keep in mind that this set is specifically for the Gan frameset.