Hi Eric,
I just tried with your suggested pinout (which I believe I have tried before), and all of them produced 8.9V! I don't think a 7805 can be easily damaged by reverse voltage - I tried applying input voltage to wrong pins with a 'normal' 7805 and it still seemed to function normally once the correct voltage is applied. Not to mention that previously I only tried a few random pieces among them, and not all of the 78DL05APs. So my 78DL05APs are somehow faulty - unbelievable but unfortunately seemed true.
What could have been the reason?