[vitovito]

Not a docking connector: charging contacts.

See page 29 of the service manual here: http://www.applerepairmanuals.com/the_manuals_are_in_here/iB...

(No third-party charger that used them was ever released AFAICT.)

[Ecco]

Amazing, thanks for that link! Do you have any idea how I could figure out what voltage to provide?

[vitovito]

I don't see that anyone's ever done the work. Disassemble it, trace the contacts, apply increasing amounts of current?

In the underside photo here, it's CP1 and CP2: https://davidigreen.com/blog/ibook-g3-clamshell-logic-board-...