It's a 5 pin DIN socket, to an RJ11 + lead with a
single large pin with a
slot in it.
I /think/ it was for a Wyse AT computer to allow the use of a standard PC
AT Keyboard, but don't quote me on that.
You are correct, that is what the adaptor is for. Or at least it looks
just like the one I had for that purpose. I'm not sure if I still have
mine or not (I no longer have the Wyse PC it went with)
I have a crappy diagram and pinout to make one of these:
<http://www.mythtech.net/WyseKey.gif>
-chris
<http://www.mythtech.net>