Dust covers are (sometimes) interchangeable. Are you sure it isn't off a Type38? The Type 38 dominated in WW1 as far as distribution. Thus, it could have been waffenamt marked later on as a reserve rifle.
HOWEVER, that's about as likely as a mummed Luger so take that theory only as unlikely BS conjecture.
My edumacated guess is that some bubba got a waffenamt stamp and viewing jap stuff as junk decided to practice his forgery skills on the dustcover.
I own firearms not to fight against my government, but to ensure I will not have to.