That little dent in the cap right where the post would be tells me It may just be a big double cap rivet. Perhaps the maker put a piece of cardboard or something between the leather layers while setting it, then removed it after to give the needed clearance. The screw heads on chicago screws frequently have ridges on the backside of the head to dig in to the leather and help prevent them from turning after install in situations where there is not supposed to be movement of the joint. You