Thank you, very thorough and informative.
So, assuming Oswald had four bullets in his clip, then the reports from the commission suggest that he fired the third shot, then cycled the bolt to chamber the fourth before abandoning the rifle and fleeing. Upon the rifle being found the last round was ejected. The clip should have dropped from the gun when Oswald cycled the bolt after the head shot. However, no report of the clip being found on the floor was made, suggesting it was still in the gun.
Are the following scenarios possible?
1: The spent shell from the last shot was ejected, but Oswald noticed his shot had blown the President's brains out and did not chamber the fourth round. The rifle was found with the bolt in the 'pulled back' position (I don't know the correct terminology) and the police officer cycled the bolt to chamber and then eject the last round. This should also have caused the clip to drop out of the gun, but again no mention of the clip doing so was made.
2: Oswald tried to chamber a fourth round and take another shot, but the 'jam-prone' Carcano jammed, and the jam resulted in the clip being loosely stuck in the rifle.
3: At the time of chambering the last round the gun was not being held horizontally, and hence the clip did not slide smoothly out, getting hung up on the way and staying in the rifle.