I never did any engineering in German, it was all rather "can you show me the way to the beach?"
Mind you, an Austrian engineer did show me around an Air Lauda plane once. Ah, 9/11 ended those days of friendliness.
Back to the structure in question; it is attached to a tube connected to the cylinder heads and Bletchley tells us that there was no mixture control on this machine. Certainly an air/fuel mix is being supplied, but I am not so sure about pressure.
Scratch all that. I've found a French diagram of the DIII (in Jane's Fighting Aircraft of World War 1, but originally from l'Aerophile) that describes this structure as a "Nourice d'eau". The water cooled radiators are on the side of the fuslage, but this structure has something to do with them. So you are correct Olham.