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If you selected the better spec MBA 11 inch and max out the specs on it when you go in the customization screen, you'll see the model change to ZOMG.
 
My BTO 15" 2.0 High-Res AG came from China as

MBP 15.4/2.0/CTO Z0LZ

However traded it in for a 2.3 15" High-Res AG and that has the normal M part number.
 
Z-spec just denotes custom built. I believe it specifies some combination of parts that are not stocked at a retail store.
 
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