The Nellhaus recently popped up as a reference chart, so I am wondering if there is anyone still using with the current WHO growth standards that should encompass a multitude of race, ethnicity questions
Place the tape on the forehead and circle around the head, passing the highest part of the backhead.
Yes, it is used specially because it goes until 18 years while the graphs provided by the WHO, only show the chart until 3 years old. Do not get me wrong, it is used in babies, but sometimes, after measuring the HC of the baby, the physician may want to measure the HC of the parents and that is where having the age-extended chart is useful. And you are right, there might be differences between those groups that you mentioned as examples. Take a look to this manuscript:
The influence of feeding patterns on head circumference among Turkish
infants during the first 6 months of life, by Mustafa Metin Donma. You see, this is basically the same experiment Nellhaus proposed in 1968 but with some specific conditioning.
Hope it helps.