Dan-San,
thanks a lot for the info, I had not heard of the red border before.
here are a 2 renderings of how the aircraft should have looked like(already with Dutch roundels), one with lozenge on the wing undersides, the other with natural linnen.Red border along the tailplane still has to be added....
Although I think the natural linnen finish is a good possibility, I still believe the blotches on the underside of the wings in the pictures belong to a lozenge covering. Any comments are welcome.
Cheers,
Richard