There is a book about Dutch DC-3 operations called "Wij vlogen naar Lissabon" (We flew to Lissabon) which has some background on this event. It is in Dutch but I could look some stuff up for you, or perhaps copy some bits if your knowledge of Dutch is sufficient. The book is a few years old by now so there won't be much in there concerning the reasons behind the incident, but I do know that the background of how KLM came to operate those flights is covered. Send me an e-mail through my site if you're interested, I don't visit here that often so may miss some posts.