A quick glance through
Militarfahrzeuge of the Wehrmacht yielded a few photos of M-B Type L 3000 S trucks, but I don't see any with flaks mounted. It's more numerous counterpart, the Opel Type 3.6 ("Blitz"), was built with "at least a hundred different bodies". In 1943, Benz phased out production of it's own designs to participate in the production of the Opel Blitz. Since these two trucks were very similar, I would think they would have been similarly modified, but I need to research this more to find a photo. Schiffer's;
Trucks of the Wehrmacht, ISBN: 0-88740-686-6, might shed some more light on this topic. Unfortunately, it's not currently in my collection.