Sounds like it may be chambered for the 20-bore brass case, I'll try hitting the books tonight for more info.
Very long barrel for a 20-bore rifle, a target-rifle perhaps? What type of rear sight?

Just curious, are the proof-marks Brit or Belgian?
Are there any?? ...should be on the side of the barrel close to the breech if British.

A bit cliche', but it could be worth trying the Martini web-sites/forums, though I'm sure someone here will have more info.