The Nortons have wide tool boxes but still with centrally mounted speedos which probably narrows things down to contracts C3655, C5109 or C5612. The practice of applying the WD Serial to the rear number plate seems to have begun during C5612.

There were no C3655 bikes ending in 2745. It could be C3932745 (W1036) or C4102745 (W5188) of C5109 but most likely, C4152754 from C5612 (W7577).