I think the intrusion bars fixed it, as my LH doors flapped at the top and when a unused set of A9X doors were put on it never happened again, the new hinges went on to so that may have helped.
That is correct there is no difference in the rigidity of the shells. The difference was in the 1977 introduction of the heavy door intrusion bars. Anyone who has picked up or re-skinned the doors with and without intrusion bars will know the difference. Especially in the big two door (hatch) doors its very significant.
Edited by LXSS350, 06 August 2013 - 03:02 PM.