Harald...for what its worth...Years ago I read a book called "The Smirnoff Story"..a biography of Ivan Smirnoff, one of KLM's first pilots. Apparently when the Netherlands was invaded KLM evacuated its operations to the UK. Their aircraft, DC3's, were used for various ops including the courier flight to Portugal. From memory it was one of their aircraft in question.
The above book may shed a little light for you ,alternatively there may be someone connected with KLM or retired (Dutch retired pilots association?)who may be able to help.
Look forward to the fruits of your labour and best of luck...Emeritus