This is the Nikon-receiving side of the adapter ring. The Nikon lens is placed into the ring on this side. A Nikkor lens's mounting lugs will fit into the large open gaps that span from 10 to 1 o'clock at the top, and from 3 to 5 o'clock on the lower right,and from 7 to 9 o'clock on the lower left side of the ring. At the top is a small hole,which has a small pin,on which you can see some brassing; that is the lock/release portion of the ring.