Sounds like you're looking for the chassis s/n which would only be in the FRU if an administrator put it in during configuration. AFAIK this is not populated by OEM. You could substitute the baseboard s/n for this though since it will be unique. Just a suggestion.
Chassis serial numbers can be found on a sticker on the chassis. For rack mount servers, the sticker is ususally on the top of the chassis. Pedestal serves the sticker is on the back of the chassis.
the problem is that I cant view the sticker as the servers are inside racks and stacked. HP has a really simple vbs script that reveals the chasis serial number in order to maintain and monitor your support contracts. I cant believe that Intel has not though of the same idea.
Also I cant locate the proper link where I can register my servers. I seem to always end in software development products registration
The only thing I can suggest to you is go through and manually enter the chassis s/n in the FRU when new assest arrive, and carry out this procedure retroactively. Of course you would have to gather the numbers manually as well, so you might not want to bother with updating the FRU.