Franz,
4403 is a nice early number, part of contract N° S1945, stipulated 02/01/43. Deliveries started November 1943, continuing into 1944. Your serial N° would be C 5111404.
By the way, a really nice 'flea you've got there. The only bits that I can see that look wrong are the headlight (Miller), headlamp brackets (were strip steel), carburetter (Villiers with strangler and air filter), twistgrip (was a flimsy affair - can post up a photo if you need it), toolbox, handlebar locking lever and vented petrol cap (Terry Jones reproduces them all). I see that you've still got the original Mansfield saddle - very rare.
All the best,
Stefano