Both these machines have been recently taken over by the Wehrmacht and are likely to be in their original form.
C3655 covered WM20 3001 - WM20 4601 with numbers split into two groups. There is a difference of 843 between these census numbers and although they were not 'married' to frame numbers, it would seem that the change probably took place between WM20 3750 and WM20 4600. Based on the factory records, this would imply that the change occurred during July 1939.
One thing though Rik, if you look closely at the second picture with the number painted straight onto the tank, you can see it originally had a square under the paint. So I would say this bike did have the black square but has been painted over with green paint and the C number reaplied. The problem with this picture is you don't know when the German's captured it so you can't say this is the original paintwork
I'm not sure that the darker area is anything more than shadow and fuel spillage. The first figure '3' seems to be outside of the area. If there has been an overpaint, could it have been of the initial Ordnance-applied black panel prior to the signwriter carrying out his task such as on this 16H ?