I have no idea about German firearms but I think your right to be sus, it just doesn't look correct, I have one similar mark on a captured Bren gun and it looks more detailed than the one you have shown.
It looks terrible, and how can you have two Z's in a row that are not even the same size? Scribbled with some electro pencil?
Very crudely done.
Books published to date... 'Belfast Diaries: A Gunner In Northern Ireland'... 'A Tough Nut To Crack - Andersonstown'... 'An Accrington Pal: The Diaries of Pte Jack Smallshaw, September 1914 To March 1919'.
An added spurious stamping....just another example of tarted up rubbish to jack up the selling price to the uninformed...funny they didn't add a Skull to it too....
"Much that once was, is lost. For none now live who remember it."