Posner's conclusion was valid, but by failing to do his homework, he failed to impeach the witnesses' credibility as he'd intended.
Posner notes that Edwin Lea McGehee did not recall the car in Clinton as a Cadillac, but as a beat-up Nash or Kaiser sedan (Ibid. 146). But McGehee did not claim to see the same car that Collins, Manchester, Dunn, and Palmer saw. Edwin Lea McGehee wasn't at the CORE registration drive; he was the barber in Jackson who said he cut Oswald's hair, and no one ever claimed that Oswald appeared in Jackson and Clinton on the same day, in the same car, with the same people.
Posner ridicules Bobbie Dedon because she "claimed she did not see the car" in her original statement (Ibid., 147). That's absolutely true: She did not see the car. She spoke to Oswald inside Jackson's East Louisiana State Hospital, where she worked -- again, not in Clinton.
Posner says that Dunn's July 13, 1967, statement alleges four men getting out of the Cadillac, one of whom was a townsperson he knew, which of course is not what William E. Dunn, Sr., said at the trial (p. 146). But Posner isn't referring to a July 13, 1967, statement of William E. Dunn, Sr., the CORE volunteer: He's referring to the July 13, 1967, statement of one Andrew Dunn, who did indeed describe four men getting out of the Cadillac, who wasn't called to testify. (An alcoholic, Dunn's story bears little resemblance to that of the other witnesses.)
Posner also errs in saying that the townsperson Andrew Dunn knew was necessarily one of the men from the Cadillac. The townsperson is Estus Morgan, and Andrew Dunn knew him personally, but he could not remember whether he saw Morgan in line to register on the same day the Cadillac appeared.
Posner also states he could not locate Estus Morgan, which is understandable, since Morgan is dead, but author Patricia Lambert, only a year after Posner's book was issued, found Morgan's widow without much trouble and learned a good deal about him.
Henry Burnell Clark -- who did not testify at the Shaw trial (Posner implies that he did) and who Posner calls "Henry Brunell" -- is described as recalling an incident involving a lone stranger in a car. Posner isn't even close: Clark had Ferrie and Shaw wandering up and down Main Street, not far from the CORE drive. There can be no doubt now that his story was wholly fabricated, though it's not clear why the State didn't use him at the trial.
Clark would be virtually unknown if journalist Hugh Aynesworth hadn't made a trip to Clinton and tried to interview him and others in 1969. Aynesworth describes this in his first-person narrative inserted into James Kirkwood's American Grotesque: "Henry Burnell Clark . . . was supposed to have identified Ferrie and Shaw on the Clinton main drag." When Aynesworth tried to talk to Clark, Clark only said, "Go 'way, man. I don't know what you're talkin' about" (Kirkwood, 222).
Bill Davy and Jim DiEugenio ask, as I once did, why these witnesses weren't called by the defense. Now at least I know one of the answers: Andrew Dunn, an alcoholic, was dead by the start of the Shaw trial (Lambert, 323 fn.). Whether any attempt was made to get Henry Clark to testify is unknown. The mere question is darkly comic, however, in a way I myself was not aware when I asked it.
Louisiana had no law of discovery entitling the defendant in a criminal trial to evidence about the State's witnesses -- or even their names. The defense would never have known the witnesses' identities in advance had Hugh Aynesworth not been able to get ahold of a "contraband" copy of Garrison's master witness list -- probably from Garrison investigator Bill Gurvich, who took a copy with him when he resigned from Garrison's investigation, and happened to be accompanying Aynesworth on his trip to Clinton. Garrison defenders to this day call Gurvich's act treasonous.
The best Posner can do with Garrison's key witnesses from East Feliciana Parish -- Henry Earl Palmer, John Manchester, Corrie Collins and William E. Dunn, Sr., -- is note that Collins thought the car's driver wore a hat (Ibid., 146); and note that registrar Palmer thought that Oswald was with Estus Morgan in the voter registration line that day (Ibid., 149), which Palmer doesn't explicitly state, although it is strongly implied. Posner doesn't even mention that Corrie Collins didn't recognize Dave Ferrie or Clay Shaw as men he saw in the black Cadillac.
Does Posner score any points? Only the same one the defense scored at the trial: US Weather Bureau records for August and September 1963 show that for Oswald to have visited with Edwin Lea McGehee and Reeves Morgan, either both men were wrong about the weather, or the date had to be later than the "cut off" point of September 24, 1963, when Oswald left Louisiana, never to return.
It was Patricia Lambert who went to Clinton and Jackson, interviewed a number of people, and turned up the missing link to the real story behind the Clinton witnesses -- forgotten Garrison investigator Anne Hundley Dischler.