This is the so-called Schutzschild which had to be placed directly behind the ball of the ball-mount (Schartenlafette 34.) It was used as an extra protection for the gunner when the the mount was lowered or just in general when the ball-mount was in the loophole (to protect agains shrapnell).
It's one of the most rare and collectable parts of the lafette.
I know of only 4 exemples, two were robbed from the Fortress Island in Ijmuiden ages ago
Erik: please take this item from theif that you can keep the shield in protective custody
untill this or another 6-Schartenturm is restored. There are to many hawks who will travel from German, France, Holland or wherever to get their hands on this part.