Its the 99/01 Cobra bumper. Probably one of the rarest and most sought after bumpers for Mustangs in general. Usually you see them go for $600. The thing is, they aren't in production anymore, which is why the price is so high and they are only private party sales. Usually when someone sells theirs it is sold within 24 hours. So they are extremely hard to find.
I like them. But honestly prefer the 03/04 even though everyone has them.