The pinout is the same as all the Duos and PowerBooks that use a "stereo headphone" plug.
Tip - serial data for intelligent PSU, n.c. for other PSUs

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On the Duos (or was it the PB 100? :?: ) this was actually the ADB Data Line, which allowed for some very interesting possibilities in terms of peripherals development. Dunno what they used it for on the later PBs, but it was an elegant arrangement . . .

. . . on the PB 100/Duos/whatever it was. [)] ]'>