The pin has unique "8" shape. Each side of the pin is inserted into the link that has "C" shape slot. The square middle link is actually a "cap" that has a screw hole to match the single screw on the back.
To prevent the pin to slides, when screwed from back, the screw draw the cap in, and "clamp" the pin.
It is unique system for sure, with the main goal is to hide the screw from sight. But in terms of practiality, i dont like it. Too many parts. More difficult to assemble by watchmaker, let alone by ourselves.
Also i am not sure about the safety. Because the screw has very short travel, if it loose just a bit, there will be no pressure at all on the cap, and the pin can easily slide out. Scary.