If your BeoSystem 2 is a (HD-ready) MK3 version you'll need:
8089000 IR-sensor with auto contrast
6277910 Wire f/IR-sensor with auto contrast (9 pin mini DIN)
MK1 and MK2 versions of the BSy 2 used:
8089110 IR-sensor without auto contrast
6270758 Wire for IR-sensor (3,5 mm mini jack)
Maybe a "BeoLink PC2 / BeoPort / Avant US" IR sensor will also work,
if your BSy has an 3,5mm IR INPUT in addition to the STB-IR OUTPUT