German soldiers putting a camouflage net over an 88 mm Flak-18 anti-aircraft gun emplacement. 

88 mm anti-aircraft battery

Young German anti-aircraft gunners (Flakhelfer, "anti-aircraft artillery assistants") from a 20-mm German antiaircraft installation (2cm Flakvierling 38) in western Germany. The young "soldier" standing, seems to wear wooden shoes.

Flakvierling

German Flak 18 cannon..
   The 37 mm Flak 18 was an anti-aircraft and and also anti-tank cannon, that saw widespread service in the Second World War. The cannon was fully automatic and effective against aircraft flying at altitudes up to 4,200 m. The cannon was produced in both towed and self-propelled versions. 
Flak is a contraction of German Flugzeugabwehrkanone, meaning "aircraft-defense cannon". 