Further to the above, it would seem that the PC input was designed to be used with BeoLink PC Office (not the later Beolink PC2 or Beoport). I would think that the extra pins in the PC socket actually gave this Beolink PC Office all the functionality of being connected to ML (and PL), without being connected to ML (or PL)! Clearly none of this was needed when the Beolink PC2 was introduced with its own ML/PL sockets.
EDIT: This from the Beolink PC Office User Manual – it’s the box nearest the computer:
