That looks like a beautiful example of a late made M97 trench gun. It even has the filed down sight bead on the barrel which is correct for a late gun like that.

The barrel date is likely 42. Try and shine a flashlight in between the barrel and mag tube to see the date. If you still can't make it out, you might have to remove the mag tube to see it.

The documentation is a big plus, IMO.

Looks like a keeper to me.