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While there is not a great shot of the rear of it, it could be a Supervettes diffuser. I have a Supervettes diffuser and the side shot does look similar.
Since the car in the pic on ebay has the supervettes rear quarter panels, then it is probably supervettes diffuser.
I am always Leary of ebay though, and the part you buy may not be the part in the picture, especially when they don't show the part uninstalled. Maybe it is just paranoia