I own a MK1 WD B40 and, barring the square area underneath, which I agree looks like a repair, my tank looks just the same as this one. There was a MK 2 version, some of which may have used a different tank, but these are very rare, only sold to a limited range of markets (Jordan springs to mind). Having said that the only pictures I've seen of a MK 2 featured the same tank...
As a point of interest lots of WDs found their way to Australia for army use. Your tank could well be off one of these. Another bike which went to Aus was the B40 'pastoral', built for farm use, which was heavily based on the B40 WD.
Cheers P