Isn't the D5 cable just a Y cable that has a male end that goes into the 33 and two female ends that allow you to plug in an the comms cable from the 512 and from say a 508??

I would just make one up and try the various wire combos in the female end, can't see it damaging anything, what voltages are we talking about over comms?

No Darling, I've had it months!