Nero,
The "Spandau Eagle" John asked about and the "Imperial Acceptance Mark" I mentioned above, are the same thing. Mine is shown in the third photo of my rifle.This is really what documents the rifle as an officially accepted sniper rifle. Considering the plaque, it is likely to have some type acceptance stamp; although in your case it may be some Bavarian acceptance stamp. It would be very interesting to find out.
Mike