Early in the movie, the four boys ride home on their bikes. Two turn left, the other two right. There you can see in a glance that the pavement for cyclist path is red coloured. This is repeated later on in a shot in front of the capital mansion. In 1960 there were no red cyclist paths part of the main road (all were in the same charcoal colour).
The story is set in 1960, the first title of the Dutch comic book series "Archie, man van staal" that the victim is reading was introduced in 1963.