you just can't get your head around the fact there never was a 'David' - the reason zero looks like one of the two little boys is IT'S THE SAME CHILD.
Next, you'll be trying to convince everyone that the darker boy is Mark, but that won't work either, because Mark was born AFTER Ruth arrived in Kenya in December 1964.
Thanks for the comparative pics. In the top picture we have Mark (2) and David (5) taken in about 1971 in Kenya. In the bottom picture we have little Barry (2) taken in about 1963 in HI, all IMO. The top picture came from the personal photo collection of Mark, IIRC. He should know whether he had a younger brother, absent a conspiracy theory.
David has a very strong resemblance due to being a brother from another mother to Barry, IMO. To me these pictures demonstrate that a father can produce two children that look nothing like the father but do look like each other, in this case each with a white mother.