Just after 9 a.m., they lifted the coffin from the earth. Kenneth Porter, Mrs. Porter's husband, said the coffin and vault appear rotted, but in good shape considering how long it had been buried.
'It appeared to be skeletal remains, somewhat deteriorated,' Porter said.
Remember that rotting is biological process that involves microorganisms, and in this case, it looks like the conditions were good for the decomposition process to take place, since even the coffin seemed rotted and the body was reduced to skeletal remains which look deteriorated to Porter's naked and untrained eye.
So, that there was a small shallow defect in the mastoid bone hardly seems surprising, and it hardly rises to the level of medical evidence of surgery. And it is greatly outweighed by the fact that neither Rose nor Groody saw the massive 3 inch scar on Oswald behind his left ear, similar to this: