Asked a friend of mine- big fan of Panzerwaffe-and this is what he told me:
The part in question must be the container for the spare antenna:
-such containers were present on both the Grille(Sd.Kfz.138/1) and the Marder Ausf.M(Sd.Kfz.138), which carried bigger guns and needed bigger(wider) cleaning rods.
-antennas were always carried/delivered in containers
-the container was funnel-shaped on the backside(towards the rear of the vehicle)so the wider antenna mount could be slid in.
-the clamps were needed to securely attach the container to the hull.