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The fact that it is stock images means it is either a used computer shop or some skell who is trying to unload his beaten iMac to some poor soul who is ignorant of its age.
 
It is possible that he swapped the whole screen to a 20"

As far as I know, there's virtually no way this could be done. The 20" is pretty dramatically different internally from the 15/17" versions and I think that the dome is larger(although I've never set them side by side). I believe the video connector is different, or if nothing else the logic board certainly is.
 
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