Wing-lights on Holland Fords similar to Harley-Davidson Knucklehead
 

Hi!
Some Holland Fords 1936-1940 and some Chevy's had popular type of a wing-lights. Probably Belgian and may be some other European too, but i don't understand this clear.
One of the pictures below shows Holland Ford 1940 with Nederlandsche-Ford emblem and typical wing-lights.

Comparing with other wing-light types I've found similar stuff on Harley-Davidson Knucklehead motorcycle (other pictures).

So, some questions i hope to clarify:
1. What was a purpose of these lights, size- or turn-indicators ? When they were standartized in road-rules in Netherlands?
2. Where such type of wing-lights were popular except Netherlands (Belgium ? Denmark ?)
3. Who was supplier ? The same as for Harley, or they just look so similar ?