The hair alone tells you that he's not Ruby. First, note that there was a persistent campaign to depict Ruby with more than he had. The image below and center is an extreme example of it, but there are many. Even the image top-left has been enhanced. Look how long the strands are. Do you really think Jack Ruby wore his hair that long? And if he did, imagine how it would look after a melee in which it got mussed. The strands would be going everywhere. So, they no doubt took out the felt pen and bolstered his coverage there as best they could. But, he still has much less coverage and much less thickness than the man on the right. The hair we see on the man on the right cannot possibly be Jack Ruby's hair. And that means that he cannot possibly be Jack Ruby.