i've had a Super Famicom for a few years, it was first owned by a japanese student who brought it over while spending a summer in the US. he had this little AV box hanging out of the nintendo AV out (the nintendo multi out not the RF).
its made up of these 3 main parts
1) the plug part, black plastic with screw down holes on the side
2)connects to a small board via 3 pins, the board is about half the size of a matchbook and is made by mitsumi
3) a metal box (rf sheild?) is soldered to the board it has one RCA type jack and a small switch next to that
i dont think its really anything special, i'm just curious if i can use it for something else or if somebody might need it, my SFC outputs perfectly over my TV so i dont know why he would have used it.