That seems pretty definitive that is a prototype "Alchemy" board for a 6360 or 6400.
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The fact its in a desktop case makes me think its more likely a prototype 6360, but I guess Apple could have been testing the faster 6400 series boards in the desktop case, maybe even considered shipping faster desktop case versions
6360/160: desktop, 603e at 160Mhz
6400/180: tower, 603e at 180Mhz
6400/180: tower, 603e at 200Mhz
Not sure if its above but what was the reported CPU speed?