1) the mortician who worked on Oswald's body in 1963, Paul Groody, was asked twice about the mastoidectomy scar, and he denied that it was there.
2) the autopsy, done by Dr. Earl Rose, the Dallas coroner, revealed no mastoidectomy scar, even though many other scars on Oswald's body, some of which were very obscure, were noted. If there was a mastoidectomy scar, don't you think Rose would have found it? The mastoid process is the big, hard, protruding bone behind your ear. When it gets infected, they remove the whole thing. It's not just a scar but a missing piece of bone. So, how was Rose going to miss that?