Notice that even at age 16, Pinkston looks pretty tall. He definitely does not look short. He's not as tall as the guy next to him, but he is nearly as tall, and he's taller than most of them.
So, contrary to what Backes said, there is really no chance that this short guy whom Oswald was talking to in the hall was Pinkston.
So, who does that leave? It leaves Bookhout and no one else. And after that, he filed into Fritz' office for the 6:30 interrogation.
So, who could that have been? He's not a Dallas detective. He's not a Secret Service agent. He is the only FBI agent who attended that interrogation, James Bookhout. So, even by a process of elimination, we get to Bookhout. And remember that he was someone with whom Oswald was having a casual conversation. Oswald goes up to him in the hall and says, "What have you got against such and such?" It was the continuation of an ongoing conversation. And, I'm sure it had nothing to do with Oswald's arrest. Oswald seems downright chatty in talking to him. And how many times have you seen this, that a murder suspect, and not just a suspect but someone who has been formally charged with two murders, is having a casual, friendly, easygoing conversation with a law enforcement officer who is involved in his arrest? I find it very strange, especially since we know Oswald was pleading innocent. That makes it a very adversarial situation, doesn't it? I wouldn't expect there to be any friendliness at all between them. But apparently, Oswald developed some rapport with Bookhout. And, that's probably why they kept Bookhout there coming back to represent the FBI at the interrogations. They could have had Hosty come back. Why didn't they? It's because Hosty rubbed Oswald the wrong way; he irritated Oswald over Hosty visiting his wife, and they must have decided that it would be better if he stayed away. Bookhout had much better rapport with Oswald, so they kept him on. This short guy is definitely Bookhout.