In the population registration 1880 - 1920 there was only one family Vlasman in Katwijk.
They were hard to find as they are not listed in the alphabetical index. I found them through the family of wife Neeltje, where there was a reference to a new volume and page behind Neeltje's name.
Johannes Vlasman and Neeltje van Duijvenvoorde, with 2 children Cornelis and HENDRIK!
If you are sure your great granddad was born in Katwijk, this must be the one.
Born Woerden 27 May 1995
Son of Pieter Vlasman and Elisabeth Boon
Married Katwijk 03 Apr 1919 to:
Neeltje van Duijvenvoorde
Born Katwijk 26 Sep 1900
Daughter of Hendrik van Duijvenvoorde and Maartje van Duijn
Cornelis Vlasman ( male)
Born Katwijk 13 July 1919
Hendrik Vlasman (male)
Born Katwijk 05 Mar 1921
Adress in 1920/1921: Louwestraat 60
Profession Johannes: fisherman
A scan of the population registration on:https://familysearch.org/pal:/MM9.3.1/TH-1951-30869-1815-61?...
bottom of the page.
It would be great if you can verify the date of birth of Hendrik or the names of his parents through the marriage licence of his marriage to Maria Hughes.