I re-used the original strain relief (rubber and metal bits) and the cable is something I found in a drawer and is of unknown origin. It is roughly the same length as a stock HD-650 cable and has a 3.5mm stereo plug at one end (with a screw thread for an adapter) and it used to terminate in two 3.5mm mono plugs at the other ends, which I cut off. It also has a nice plastic block just under chin level to prevent the wires from separating further... It's made of pretty substantial copper and looks like it was made for the job!
Sounds like an old Senn HD2xx cable.