It would be great if you could get the name of your great great grandad, the name of the father of Hendrik senior....
I did some searching on the name of Vlasman in Katwijk, and found a marriage of a Johannes Vlasman in 1919. I will pull the population registration tomorrow. It runs up to 1920, so maybe we get lucky and he had a son named Hendrik born in 1919 or 1920.
For now, good night and I will let you know tomorrow what I have found.