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Nevetas, the original "bug-eye" new age Impreza WRX was introduced in late 2000, as the 2001 model year. Thus, if you're only talking about WRXes, you'll see some X-platers around.
If you're confining yourself to the STi Type UK, that was introduced in the 2002 model year revision, with the first examples hitting the road in Jan-Feb '02. Thus, there should indeed be a few on "51" plates.