It's called the "IR code convertor" (available from 1989 - 19??)
It is a little box with an MCL-type IR-eye (the rectangular one) and an MCL cable, which is connected to SPEAKER 2 on the Beomaster. It used the extra pins on speaker 2 to control the Beomaster.
The little box has the following connections: TAPE, CD, IR-out, passive speaker out (left/right)
It can control the following devices:
Beocenter 7000, 7002, 7007, 7700
Beomaster 5000, 6000, 8000
Beovision M20, MX2000
Beovision 5502, 7702, 7802, 8802, 8902, 7102, 8102, 5500, 7700, 8800, 9000