This accessory consists of an adapter that screws into your eyepiece and two cords coming off the adapter's left side: the top one is connected to your camera's PC socket, and the bottom one has a pronged plug to go into your speedlight. Thus it also handles camera-flash sync, obviating the need for the SC-5/6/7.
This accessory slides around the F/F2 shoe and connects with the ready-light contact built into the left side of F2 prisms. It connects to the three-prong sync socket on Nikon Speedlites and also handles the camera-flash sync (i.e. no need for a PC plug cord).
The SC-5 is shorter than the SC-6, lacks the coiling, but performs the same function: one end is a PC plug, the other is the standard three-prong plug for flash sync.
It seems pretty simple: one end has a two-prong plug that looks vaguely like a US AC socket-plug (but goes into the three slots on the SB-2/3 and -7E/8E) and the other end is a threaded PC plug (finally, the threads on your F2 have some use, hallelujah!).
This is for use with cameras and other objects that have an ISO shoe without flash coupling (i.e. cold shoes); it has the three prongs which mate with the three slots on the bottom of the SB-2/3 and -7E/8E on one end and a standard PC plug on the other.
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