OK, I figured it out, and the plot thickens. But, I'll state first, again, that I am absolutely sure that that muzzle flash is fake.
But, what I noticed, upon close examination, is that the frame I used of the flash-less presentation is slightly off. Ruby is moving in from the right, and his hand (with the gun) is getting progressively closer to Graves' hand, which we can see.
And I noticed that Ruby's hand is closer to Graves' hand in the top frame than the bottom frame.
So, that made me realize that I stopped the video a frame or two early, that I had to let it run a frame or two longer in order to get an exact match.
So, I went back, and I slowed the video in Google Chrome. And I discovered that it goes from this frame, which is the same as the one I had before:
to this frame, in which you don't see Grave's hand at all.
I am absolutely certain that there is nothing in-between those two frames. In fact, what I discovered is that the film freezes on the first frame for several frames because for several clicks of the mouse, the view doesn't change; it looks the same; it looks like this:
And then, it jumps to this:
Believe me, I tried it several times; I tried it many times. There is nothing in-between. So, let's make a new collage:
Holy Cow! To my eyes that looks pretty damn good in terms of the placement of Ruby's hand. But, obviously, we don't see the muzzle flash, and we don't see Graves' hand either. And come to think of it, we don't see Oswald's clasped hands either. So, did they deliberately darken this for some reason? Or was there another frame in-between which had the muzzle flash? Did they take the muzzle flash out? Well, if so, it could only be because they put the muzzle flash in, and then they were unhappy with it, that they felt nervous and uneasy about it. Maybe they realized that the muzzle flash shows that the blast has already occurred, yet nobody is reacting to it, including and especially Oswald.
Notice that there is not one face in that whole crowd which is showing reaction to a gun blast, and notice that Oswald seems totally unfazed even though he has been shot. So, that is probably why they got rid of that phony muzzle flash.
The muzzle flash is fake- that I'm sure of. But, what I want to know is: who took that film, and why are they keeping it a secret?