I bought a C64 lot locally (C64, monitor, disk drive, games, etc.) and this cable came in the lot. It looks mass produced, so I think that it isn't custom, but it is possible that it wasn't intended for a C64 specifically.
It has 1 yellow and 1 red RCA connector and a 5 pin DIN on the other side. The first odd thing about it is that the yellow is AUDIO and the red is VIDEO (definitely not the standard in the US.... maybe elsewhere?).
Anyway, I haven't had time to build my custom cable, but I did a quick check with my multimeter and got these results:
The audio RCA is grounded correctly and the tip is connected to the audio pin like it should be, however, the video RCA connector connects to both the video pin and the 12V pin (I'm lucky nothing got fried!)
So that means on the C64 the composite video and S-video luma are connected when using this cable... it doesn't seem to have that bad of an effect on it, but I'm more of a sound guy than a visual guy anyway... so I probably wouldn't notice a difference.
That's a cool little sheet you have there... I might hold on to this.