Why don't these iPads have the IMEI printed on the back?

According to http://support.apple.com/kb/ht4061, both the serial number and the IMEI should be printed on the back of an iPad. I have several dozen MD522X/A's (iPad 4 WiFi + cellular), and they only have the serial number on the back.

Does anyone else have an iPad 4 with cellular with no IMEI number printed on it?

I can get the IMEI number from About, but not if the iPad isn't working. Is there anywhere else on it that it might be printed? I know it's on the box, but I don't want to have to cross check with the serial number to make sure I've got the right one.

Can anyone tell me which of these numbers Apple uses to identify tablets sent in for warranty repairs? I know Blackberry use the IMEI, and the serial number is useless for tracking device history with those, so I want to get this right before I've got too many of them to deal with.