Out of curiosity: Did you try the combination +5V on pin 6 (datalink) and -5V on pin 7 (datalink shield)? I have had some thought about this and cam to the conclusion that the rest of the pins are either not conencted or used for the sound part. Furthermore one of the olde manuals regarding datalink signals mentions high signals in the voltage range 2.1 to 5.5V. But again it is just a thought.
Great minds think alike! I came to the same conclusion about pin 6 and did try 5v to that with pin 7 as ground (not -5V). I was actually using 4.5V (3 x 1.5v cells). Mine is SW2.0 whereas Matador’s picture from the one that worked says SW1.1 – perhaps that’s the problem?