Oswald in the doorway: the blog of the Oswald Innocence Campaign, by Ralph Cinque

There were two Ruby figures in that garage, and neither one was Jack Ruby, and there is no doubt about it. It was the short one who rushed Oswald and fired the shot, and he was FBI Agent James Bookhout. I don't know who the other one was. But, did they plan in advance to do it this way? 

Maybe, but not necessarily. They may have just noticed afterwards that that other guy could be converted into a Ruby figure. Perhaps they came to think that it was no good that "Ruby" wasn't seen at all prior to the shooting.  And of course, they couldn't show Bookhout because, well, he was Bookhout. They couldn't show his face at all; not before, not during, and not after. There is one picture of him after posing with Detective Boyd, Sims, and Hall, but IT WAS TAKEN BY MISTAKE, and the mistake was compounded when Fred Rhinestein included it in the 30 year retrospective.

And, I'd be willing to bet anything that if they could go back and time and take this out, they would. It's obviously not Jack Ruby.

So, it's possible that they came up with the idea of making that other guy, Sunglasses Ruby, into "Ruby" after the fact. 

That image is so extremely manipulated, who knows what that guy really looked like.  If they planned it in advance, I should think they would have gotten someone who didn't require so much work and who was the right height. So, this whole thing may have been an afterthought, and it probably was.