Hi Ivan,
It really is an interesting bike you have there!
Make sure you get a copy of the factory ledger, so you know exactly what it does and doesn't say. If you wish to try to return the bike to its original condtion, you then have an interesting task.
First, the bike may have been fitted with Webb forks in 1925, but are these the forks still on the bike? Find out exactly what the 1925 Webb fork looked like (the early ones are sure to have some peculiarities not found on the later ones), and compare it to what you have.
Note that the works record doesn't say "modified frame". We could probably assume that the bike started off with a standard frame. In this case, the extra-large tank would look much like the standard tank but wider. I'm not sure how many flat tank Norton's you've seen, but some (particularly the Big Fours) have very, very wide tanks! You usually don't notice in side-on photos, but sitting on the bike your knees are wide. My guess that the original tank would have looked much like a standard 16H tank from the side, but wider.
As I've mentioned before, don't rule out using it "as is" before you decide exactly what to do with it in the future.
Leon