Thanks so much for the prompt response! I knew about "Tienjarige tafels"; but had not considered the possibility that these might have been scanned for periods when actual registrations have not. Although 1913-1922 has BAX births (I see only 3 not 4, next page is a different surname); the fact that there are no BAX births 1903-1912 in Zevenhuizen means the older boys were born somewhere else; probably Hillegersberg. Too busy today and tomorrow; but will be looking for those soon!
Births 1903-1912 no BAX entrieshttps://www.familysearch.org/ark:/61903/3:1:939K-SJ1X-V
Births 1913-1922 https://www.familysearch.org/ark:/61903/3:1:939K-F8Q4-F5
Hendrik 10 may 1916
Simon Pieter 16 Jun 1914
Wilhelm Maria 18 Nov 1918