Hi Dave,
That's an unusual underlever model that I have not seen in real life, but I doubt Mayer and Grammelspacher would have fitted a rival Walther rear sight unit when they already had their own types in production, so I would guess this part of the rifle at least was not original ( sorry about the pun - it's not Diana either!).
I will check to see if I can find anything more about this rifle, and if I can I will pass it on to you.
Do you have any pictures to share with us - I would love to see some.