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The large plug on the back is for an Audio-video monitor. It also
passes ADB for mice/keyboards. The monitors were monsters and almost
no one uses them. I have a bunch of the adapters that allow you to use
a "standard" Apple monitor, or to which you can attach a VGA adapter and
then use a generic PC monitor. Depending on where you are, I can
probably mail you one for $7.00.
Ken
http://mysite.verizon.net/res7gt1w/stackomacs