Looks like pins 19-21 are the S-Video pins. Again, whether or not the AV Macs can use it, I don't know. Seeing as the AV x100-series Macs have actual S-Video ports on a physically separate card from the onboard video, I'm not certain Apple would have put the connection in from the motherboard to the PDS in such a way that this onboard-video-in would pass through properly to the PDS card video in.

Unfortunately, my x100-series AV systems are all in storage at the moment. (Although my AV14 display is reachable...)