I spent 2 hours reading manuals. No, it is not the manual. That is why I asked. The DSI Cable Manual kinda identifies pins 4, 5, 1, and 8 as doing something, but not what they do.
After some experimenting the answer is pin 4 = D+, pin 5 = D-. I suspect pin 4 is jumpered to pin 1 and pin 5 is jumpered to pin 8 for the purpose of daisy-chaining.