There's only one slot missing on the M16A2/M4 flash hider, to prevent vented muzzle gasses from kicking up dust when fired from the prone position. You sure that hodgepodge M16A1 lower/M16A2 upper doesn't have an M16A1 flashhider on it?
The picture I saw was the same guy, but a different photo. There was no mistaking the vent on the bottom.