Yeah, it's a 2x2 plug. It was added to the ATX spec later because the Pentium 4's require so much power, they get their own plug. Your power supply should have a matching plug.
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Andy - Reinstated
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'06 Subaru Legacy Spec B - Stock, for now
'98 Civic EX - CTR headlights and grill, Kosei K1's, for sale
'90 240SX - SR20DET that will never get installed, project car.