I've heard that particular port is for video (I could be wrong) but I'm pretty sure I read it here on the forums.
Quote from someone else on a different forum
Damn, no one bet.
Anyhow it is confirmed, it is an RGB socket and I am sure it WAS supposed to be a high-density 15-pin D-sub (VGA).
Analogic will sell you the cable with the unit for an extra £15 on top. Or you can make your own

What is more fascinating is that these were designed and built by the same guy who created the BoXer and the Access, Mick Tinker for the Wall Street Institute and hence the name CD32fWSI.