I would say that your latch might be to blame. In my newport I have to use the key to lock it but in my scamp I can lock it then close the door. Maybe someone put the wrong latch inside? or hooked it up wrong?
I believe they changed it in the early 70's to be able to lock the door without the key. My 70 duster had to be locked with the key as well as all my pre 70 mopars.
