Something unusual here. It's pretty apparently part of the 4700 system, and the 4704 did have pin pad capability, but there is no mini-DIN connector on a 4704 base unit (I have one).
If I were to take a guess (only a somewhat, and self-educated one) it would be that this does plug into a normal PS/2 port (though may or may not speak traditionally useful scancodes) and was used for one of the Personal System/2 computers that could be up-optioned to function with that system.