In this case, if we look at his right hand, you can see that the thumb is out of it; it's not visible. On his left hand, we can see his thumb, but just imagine if you were looking at him from behind as in the Jackson photo. You wouldn't see his thumb. It would be tucked out of the way. Well, that's the situation with the left hand in the Jackson photo. The thumb is tucked away, and they just added that other thumb. What Bookhout was doing was making a fist.
And notice that the thumb is rising not from the wrist but from above the wrist. But, everything beyond the wrist arises from the wrist. It has to. It has to go from the wrist to the hand. But in that image, you can clearly see the margins of the wrist on both sides. And, the thumb is rising from above the wrist. Why'd they do it? Perhaps just because his hands were small. But, it is really quite ridiculous to think that his pinkie would be flexed more than his other fingers.
You're seeing all the proximal interphalangeal joints there, and they are all lined up, including the one for the pinkie, and that's typical, especially for a spontaneous fist, meaning one that you make without consciously thinking about it too much.
The idea of that, I believe, is to rationalize why we don't see the ring on his pinkie, as we did in Beers. That's supposed to represent the pinkie being curled up so much that we don't see the hulk of a ring that was on it. But, I don't think that is tenable, and the inferior edge of that hand looks strange and inexplicable. Nobody could duplicate it photographically. We really are looking at a bizarre alteration, the purpose of which was to hide Bookhout's small hands and his lack of a ring.