What is this cable for?

In a second hand shop today I bought one of these for fifty cents australian, thought I might be able to use it with my mega drive. There is one 5-pin DIN connected to four RCA plugs, coloured red, yellow, black and white. I'm assuming that two of them are for audio, and perhaps one for video. What does the fourth one do? Sync?

I don't think that's a video cable. From your description, it's a stereo audio cable that works in both directions. Older hi-fi systems and reel-to-reel tape recorders would connect this way to allow both playback and recording. I've seen this a lot in Germany, where I used to live.

So two of the RCA jacks are for audio out and the other two for audio in.
Thanks for that. I opened up the DIN to see whether I may be able to use it on my mega drive one, and lo, the black RCA corresponds to composite video and red to the mono. So it wasn't a waste of money after all
Cool, I'm glad you can use the cable. But what did you mean by "the mono"?

Are you saying that you get an S-VHS signal (brightness on one wire, color on the other)? Or a standard composite video signal on one wire, and the same signal in black-and-white on the other?
Ugh. Consoles with monaural audio.
I was under the impression the Commodore 64 and NES were pretty much the last systems to commit this heinous crime.