Reply #15: The records are probably dated close to the meetings.

[FarCenter]

The excerpt from Pannetta's letter in the image attachment to the OP says --

"This information, however, is drawn from the past files of the CIA and represents MFRs completed at the time and notes that summarize the best recollections of those individuals."

I would expect this means that the dates on the MFRs are the same or close to the dates of the briefings. The dates and integrity of the records should be attested to by the record custodians. Beyond that you would have to call the authors and briefing participants as witnesses individually, get their testimony as to the veracity of the records, and then compare their testimony.