Ho ho Pim: I just bought some Koolhoven books and the prop was hanging for more then 70 year above a bar at Nieuwenhoorn, only 25 km's away from Koolhoven plant Maasvlakte: clear evidence it's a Koolhoven prop!
:-))

Serious: thanks and thanks also in advance for possible more help. I will search about these engines in what planes and behind what props they were used. But given only 80 HP (if correct), I guess it must be planes before 1915-25. The finding of (1 prop in Alkmaar 2 weeks ago and) an identical prop so close to Maasvlakte: still possible a young Koolhoven plane? (K. used some Gnome's)