I'm probably out of my element here. I'm not a researcher in the case, I have not read/studied anywhere near as much as the rest of you folks. But neither do I have an agenda.
But from what I have read...
Pertaining SOLELY to the placard...
Was it an internal type or external type placard? From what I have read, that has not been definitively determined.
If it was an internal type placard - Did it come off the night of the hijacking, or did it come off during the subsequent test flight. From what I've read, it was not noticed missing until after the test flight. Was the flight path, altitude, and winds identical during the two flights?
If it was an external type placard - It could have come from any plane at any time.
This leads me to believe that the placard can NOT be used to definitively confirm anything.