BTW, offhandedly this rifle reminded me of a repeated promise related to stormtroopers, - when I see such rifles it reminds me of the origins of the doctrine or at least its German implementation. I have long been a part-time bookseller, 30 years or more, and longer still a book collector (as a kid I asked for books for Christmas!)
Anyway, during my ebay days I haunted German ebay looking for accessories, company advertising and literature etc... and I came across a periodical from July 1916 displaying a very common picture often attributed to 1918. The periodical is dated 1916 so the picture date could not be 1918 era. Your rifle reminded me of this picture and the possibility that a rifle such as yours could have been a trophy from a brave young man like pictured below.
*** it took me over an hour on my old computer database to locate this series of pictures, - often my failed promises of database help is related to the fact my main research is on a computer I bought in 2002 and my current computer is a gift from MikeF, otherwise I wouldn't even be online... which would be welcome if not for this forum. I hate modernity and the internet... I love books!
