Hi Raimey,
I've just taken some extra photos, the muzzle doesn't look that menacing, it just has the same ribbed detailing to the rib between the tubes and a tiny bead sight;
https://flic.kr/p/2ic3gRGI think I've managed to photograph the load details too;
https://flic.kr/p/2ic2e31There are some additional marks on the tang that the forend mounts on which I hadn't noticed before, its like a stylised 'X' on the top face and some numbers or letters; S . S . F ? or S . S . u? and possibly two numbers slightly fainter to the left hand side 33?
https://flic.kr/p/2ibYSFRhttps://flic.kr/p/2ic2e4yI've a friend who speaks fluent German so I'll see if we can send Herr Haggard an email.
thanks for your time on this, it's been great learning so much so quickly!