My trusty old Packard Bell 810 motherboard has what appears to be an unpopulated section for a ATX PSU connector right next to the (populated) P8/P9 connector(s). I never really thought to check it before, but while doing some continuity checks on the unpopulated section against the P8/P9 connector, I found a couple of odd things: the +12V pad only has continuity for a few seconds and won't have continuity for some time afterwards (I'm not sure if this is a capacitor or something causing this) and also seemed to have continuity with -5V, but even more strange was the -5V and -12V seem to be swapped.

While looking at ATX PSU pinouts, I did notice that there are some "ATX" 20-pin PSU connectors that supposedly have very different pinouts compared to the actual ATX standard, so I wasn't sure if this was one of those cases or if this board was produced before the ATX standard was fully finalized (or maybe the pins were swapped in some pre-1.0 revision?) and so they just guessed (wrong) at the pinout and thus continued to use the old P8/P9 connector(s) instead?

The only reason I'm bothering with this is that it would be nice to be able to plug in an ATX PSU directly, use a regular ATX power button, etc. without any adapters and such. With that in mind, I'm not sure how safe it would be to remove the P8/P9 and populate the ATX connector myself and use it as-is.

Here's the motherboard PSU connector:

For reference, here's the ATX pinout oriented in the same position as in the picture above with my noted changes:

Has anyone run into this sort of thing before? Am I just doing or measuring something wrong?