Oswald's on second in the lunchroom. But what was he actually doing there when Baker spotted him?
At first Truly did the Evolution Two-Step to evade the question. In a series of statements, he simply has him "in" the lunchroom; specific location and activity are left to the imagination. Then, pretty quickly, Truly has Oswald sitting at one of the tables.
Within a week or so, Oswald is brought to his feet. Truly describes him, first, as leaning against the counter, and then as standing right over by the Coke machine.
Why the Coke machine? Because Oswald had talked about purchasing a Coke before the assassination, and having him over by the Coke machine turned this into a cool-as-a-cucumber post-assassination act.
But there was a second reason.
In February 1964, French writer Leo Sauvage called Roy Truly and grilled him about the lunchroom incident. Truly revealed the game plan as he and Baker were getting ready for their meet up with the Warren Commission a few weeks later in March: the officer (name still unknown to Truly!) evidently had heard a noise coming from the lunchroom, the noise, evidently, of a coke machine delivering up its product to the man who had just shot the President.
This was a crucial addition, for it gave Baker a reason for checking out the lunchroom—a reason he badly needed—as the lunchroom was nowhere near being in his line of sight as he came off the landing.
Just look how far he would have had to swing over to the right to get a line into the lunchroom:
The door of the lunchroom being open, the "cluck-clunk" noise of the Coke machine would have been heard by Baker. Except it...wouldn't've. For there was another door between Baker and the coke machine, and it was an automatically self-closing door.
Washington Post, 12.1.1963
This news story seemed beautifully clean and convincing. However its lack of acoustic plausibility meant that a further evolutionary step would be necessary.