This excerpt from "The Tippit Case in the New Millennium" by James DiEugenio
"As if the witness censorship and the oddities with the ballistics evidence were not suspect enough, let us now turn to the radio transcripts. As most of us know, there were three versions of the radio messages that were eventually delivered to the Warren Commission. Sylvia Meagher pointed out that the first transcript the police turned over did not include what was perhaps the most important instruction given to Tippit that day. That was the order for him to move into central Oak Cliff at 12:45 PM. (Meagher, p. 260) This was about 15 minutes after President Kennedy was killed ... just prior to this message, the dispatcher had said, “Attention all squads, report to downtown area Code 3 to Elm and Houston with caution.” (WCE 705, p. 397) Which, of course, makes perfect sense. (The police tried to disguise this order in the first transcript, but the FBI actually listened to the tapes and this was their accurate transcription. Police Chief Jesse Curry confirmed this in a letter to the Warren Commission, see WCD 1259, p. 3)
...On the transcripts, there is no acknowledgement of this order, even though it was given to two men, Tippit and R. C. Nelson. (WCE 1974, p. 26)...
We now come to the message at 1:08. Between 12:45 and that time there were four messages involving Tippit. Three went from the dispatcher to the patrolman and one allegedly came in from Tippit; this was at 1:08. In one version of the messages, Tippit called the dispatcher twice and got no answer. In the FBI version, Tippit’s call number at that time, 78, is missing: the message is assigned to No. 488. The FBI notes the sound is garbled and No. 488 is not identified by his name, and there is no other message that is assigned to whoever this person is. Tippit researcher Bill Drenas, who wrote an interesting essay, “Car #10 Where are You?”, did a thorough examination of the tape and transcripts. He concluded that the voice is not Tippit’s either. But he disagrees with who the real caller is. He says it is call number 388. Which would be from the Criminal Investigative Division."
This suggests a deliberate attempt to alter the transcripts.