What makes sense to me is the agent made an error of the testing date. they seemed to have evidence of the placard being outside but that doesn't make sense either.
Prior to the January testing the Air Force was going to jump from the plane. I'm not sure on the date but it was 1971. we do have evidence they went back into the plane several days later to look behind the panels in the stairwell to see if Cooper could of hid behind them.