Found this picture, I don't think it is a gun/mortar. Could it be a dummy weapon or is it a sort of periscope/camera (equal the "fernkamera")? What is the origin country of this thing (Germany?)
"Sometimes things that are not true are included in Wikipedia. While at first glance that may appear like a very great problem for Wikipedia, in reality is it not. In fact, it's a good thing." - Wikipedia.
What about a "Erdmörser" (sort of handmade Ehrhardt mortar), German variant of the French "Sceaux a charbon".?
Forumeerstewereldoorlog says the erdmörser is a wooden "mortar" shooting pots like below. The explosives were "Black powder" initiated by electric wire or just with matches. The thing the German is looking at is maybe the fuze wire.
This is just my thought and is not based on any historical notes
I remember reading an article about training trenches in the UK, often dug by troops home from the trenches. While not training up new soldiors i believe some were opened up to the public for them to get a feel of the front line.