Nobody got a bit of advice on what to do here?

Use a multimeter to find out which side has 0 ohm continuity to ground. That's the - battery terminal. + is on the opposite side. The 2 center pins are N/C as you suspected.

greetings!

That GIF you linked to is much better than nothing in general, but at least "fairly mainstream" boards like this one will have a "real" manual.
Therefore, did you consider googeling the search string "chaintech 486sle"?
Because that is what i found:
https://archive.org/details/chaintech-486-sle … age/n7/mode/2up

Page 15, on the right: There's your pinout.

If not for that archived manual, an approach would have been:
a) look for a somewhat similar board by the same manufacturer, they don't reinvent the wheel all the time.
or
b) assume that it's a connector like most of the others, where Vcc is the first and ground is the last pin - use a multimeter or squint at the traces to confirm which is which.

Concerning your questions:
- Some motherboards will have a jumper to select onboard vs. external battery, but apparently not this one.
- With some you'll need to make sure not to damage a non-rechargeble battery by connecting it instead of a rechargable one, maybe by adding a diode in line. But not this one, I'd say.
- Manual asks for 4.5 up to 6 Volts which is rather strange als most only use 3 V and most battery packs for motherboards are for three cells, which only is about 4.5 for fresh non-rechargeble or roughly 3.9 for NiMH cells.
But try it with three cells and it probably will work. Or get two lithium buttons like cr2032 in line for 6 V.
There are rather expensive lithium based AA batteries, if you want a long time solution.

Have fun!

Sorry for not replying sooner. Had no option to test for a while.
I used both of your advices. Checked in the manual and them also used to multimeter to confirm that it's correct, and then tried it. And it works!
Unfortunately I stumbled over other issues then, but thats a different story. ^^
Thanks again!