Hi Peter,
Douglas seemed to make a habit of fitting non-colour-matched items - Mark headlights were black, irrespective of the colour of the rest of the machine - the same applies to the Dragonfly stand and rear brake pedal.
Yes, that is the correct numberplate but the light is a later model than that originally fitted. The fact that the Miller light doesn't have a stoplight is not a big problem. If you take the innards out of the lamp holder, you can replace it with bits from a Lucas stop/tail lamp - then you just need to file off one of the staggered pins from the bulb (the original bulb had 2 pins in line), and then just make sure you fit the bulb the right way up!
Regards,
Eddie.