I see several connectors in that picture: the one at the top left is the Duo Dock/MiniDock/MicroDock port, standard on all PowerBook Duos. In the top center is the power jack. In the top right is the AppleTalk jack for printer/localtalk. In the center of the motherboard is some sort of ZIF connector, which would likely be used for either the mouse button and ball or keyboard. Finally, at the bottom right is the battery connection terminals. On certain models, in the top far left would be a modem jack, but this doesn't seem to be a common feature selected on the 210s as most of the ones I've seen including my own don't have it. After all, the dock itself had a modem...
I own and am pretty sure I've disassembled and examined the logic board of every Duo model save the 210. I don't recall ever having seen a ribbon cable connector on the underside of a logic board. In every case the Keyboard/TrackWhatever connection is made on the component side in an easily accessible location.
Having either connection made on the underside makes no sense in terms of documented assembly/disassembly procedures for the Duo Series. There's no unobstructed, strain relief/cable severance protection capable pathway from peripherals to the underside of an assembled Duo that I can see.
Has anyone else got a 210 for comparison of the boards?
Huh, that's curious. I wonder if it's some kind of *cough* diagnostic port. The 210 was the first model and maybe early runs needed some debugging. Then again, there's probably no reason that couldn't be done via the PDS/Docking port.
I'll have to keep an eye out for it when I start in on stripping and reconditioning my Duo stack.