Im a chevelle guy but here is what I found doing a search; Not sure the W is correct since it appears in what would be the first group, likely has something to do with the windshield.
W= Passenger Side Exterior Mirror
P= U63 Push Button Radio
X= Z = Z03 Super Sport (note 2)
(note 2) The Oakland plant used X for the Super Sport Package.
Since the tag shows the OA for the Oakland plant it seems the X identifies it is an SS.
The source is:
http://www.novaresource.org/c63.htm
hope this helps, Andy