The kidnapper named "Rem Hubrechts" was actually named Willem Holleeder.
There actually was a fifth kidnapper, not shown in the movie, called Martin "Remmetje" Erkamps.
They used his nickname and gave it to Hubrechts because they couldn't use the name Willem Holleeder because he is still around and threatened with a law suit if they used his name in the movie.
When Alfred Heineken (Rutger Hauer) is liberated in the Nissen hut, one of the Policemen is armed with an AK 47. This weapon was never used by Dutch Police or armed forces.
The weapon mentioned above is not an AK-47 but an M2 carbine. They had a curved magazine like the AK... The insignia on the berets are not correct. These did not yet exist in 1983.
Rem's (officially Willem Holleeder) nickname was 'the nose', for his obviously large nose.
Cor van Hout's nickname was 'flipper', a Dutch name for playing the pinball machine, which he liked playing.
Frans Meijer's nickname was 'stekel' (spike).
Jan Boellaard's nickname was 'de poes' (pussycat), he used to tilt his head when listening to others.
Heineken's driver Ab Doderer was the brother of a well known Dutch actor and comedian Joop Doderer.