I obtained a Mac Pro 4,1 and checked the serial number with About This Mac. It doesn't show a serial number. I have never seen this before on a Mac Pro. Does anyone have any idea what this might mean? I would be interested in learning more about serial numbers on motherboards, which apparently match the serial number appearing on the case. Is there anything that needs to be done (for legal reasons or otherwise) when replacing the motherboard? When that is done, the serial number on the case will, of course, not match the serial number that comes up in About This Mac.