One little catch: While it does have the pen, it lacks both the serial/power Y-cable. Which shares a single RJ-45 socket for both. Also lacks the power brick. And I can't find the pinout for the RJ-45 socket. :-/
Closest i've found is http://www.gtco.ca/Summagraphics/tech_docs/mm_io_square_io_ps_cable.pdf , which doesn't really help because I have no idea which color goes where in the plug. (Doesn't help that an RJ-45 plug has 8 places and they show less than 8 wires, either.) Also not sure why they used a 4-pin connector to supply one voltage (12VDC 200mA according to the sticker on the back).
Anyone have a SummaSketch III and can at least check what color wires are where in the plug? I could probably fake it from there I guess, although the schematic I linked to doesn't show what pin(s) from the power brick are 12V and which is/are ground(s).