Beoworld to the rescue !
It's the one with the metal arm pointing towards the rear of the Beogram in this photo. The arms are to the left (covered in dust on this poor Beogram). the transformer module is to the right, photo is taken from above with the black metal cover removed from under the arms.
The plastic tab protruding from the left is on the carriage (which is not quite home in the photo). The tab will activate the switch's metal arm to indicate the home position (which is where the arms rest when the Beogram is not playing).
Due to the way the Beogram counts, the adjustment of this switch will set the point of lowering the arm.
Martin