That's nice Rob. Presumably quite a late-war photo, judging by the badging. Bike looks to be a 1934 (the hub dust covers kept the water in and were dropped after a year). The silencer fits for '34 too.
I don't think that it's a cradle frame and the engine doesn't look tall enough for a 600 so my guess is a Model 18.
The rebound check springs first appeared on the Internationals for 1933 (these were the larger diameter parallel type which they retained).
The rest of the range gained the barrel shaped springs for 1934.
Norton had used Webbs for some years and I believe there is some suggestion that they were trying to distance themselves from the Webb patents and overall design. In some ways the Norton fork is inferior (not bushed and fiddlier to adjust) but the rebound springs give it an edge in use.