Thanks for the closeup of the connector, JDW. I checked around the web to see if my memory was totally gone (it probably is). The ST506 interface pinout seems to match what you've got, from the few grounds that I could see in the photo. That squares with my shaky memory.

The Wikipedia entry (http://en.wikipedia.org/wiki/ST-506) has the pinout; maybe, if you ever feel so inclined, you can check if it seems to plausibly match what you see.