No, it seems to be some sort of chin spoiler that hooks on the the factory bumper. I don't know what one it is. you might try looking up chin spoiler/ chin lip for mustangs.
Right, it's in a post '98 catalog. Mine are not handy (filled away), so I looked in a 2005 version that is and the '99-'04 front splitter was still being listed as of '05.
For the poster... Also listed in the catalog are rear bumper "spats" (no photo) and stock side scoop inserts (no photo).
I think that blue car posted above is the first one I've seen on a vehicle.