Let's start with the idea that Babushka Lady took the Moorman photo. Well, BL was shooting from a different angle than MM. BL's perspective was very similar to that of Marie Muchmore, except that Marie was much farther away. But, they were pretty close in terms of the line of sight.
So, if we look at what Marie Muchmore captured, we'll see approximately what Babushka Lady captured.
This Muchmore frame is very very close in time to the Moorman photo. Note that Mary Moorman does NOT have her camera up to her eye. Her hands aren't high enough. So, she is not shooting a picture. She has already shot her picture. But, Babushka is shooting. And notice that when it comes to the motorcycle cops, we are mostly seeing BJ Martin. That's his bike that we are seeing. All we are seeing of Hargis is his white helmet, his neck, and the top of his shoulders. Note also that BJ Martin appears to be ever so slightly in front of Hargis, even though he was really behind him. That's due to the parallax effect from the angle of view. But, it would have been the same or similar for Babushka Lady.
So, when they altered BL's image to turn it into MM's, this is what they must have started with:
But, Mary's photo was taken perpendicular, which means there was no parallax effect; she captured it the way it was, with Hargis in front of Martin.
So, they had to duplicate that, and that's where the thumbprint was useful.
There are two bikes there appearing as one. Look at the front wheel and fender. Now look at the rear tool box behind Hargis. They are not connected; they are not the same bike. The front wheel and fender are close to us, closer to the curb, and less far out into the street. The rear tool box is deeper: deeper in the picture and deeper in the street. The two are not connected. Look at this photo:
Finally, the image of BJ Martin that appears in the Moorman photo, consisting of a partial handlebar, his hand, and his forearm, is completely bogus. It's not photography; it's art. And it's very crude art at that.
That is not photography. That is not a hand, and it is not a glove either because Martin wasn't wearing gloves. It looks like a clam. All of that fake imagery was added to the photo to duplicate how Mary Moorman's photo must have looked.
So, that is what they needed the thumbprint for, to completely take out the image of BJ Martin, who was perched slightly in front of Hargis, just as in the Muchmore frame.
That is how I see the Moorman photo. Here is Martin's back: