I've got the speccy version and I believe that the moulded joystick plug contains a digital to analogue converter from what I remember reading when I bought it.
The 3 pins being used on the joystick plug are 1, 6 and 8, the only pin that gives a reading in No 8 and that is around 450 ohms on the negative of the headphone plug and between 1400-1500 ohms on left and right.
Although the CD contains 735MB according to Nero, it actually shows up a 0bytes in my computer, it plays ok using media player etc which also show up the tracks (maybe just my drive)
Here's a couple of pics off my phone of the doings, don't blame the camera, I am a crap photographer
I should have also mentioned that the booklet is for both the spectrum and commodore 64, from the looks of those other auctions that have been mentioned (C64), the cable is the same as mine to at least look at.