Licensed to kill,
If you apply pressure to the small piece behind the "cross notch" and if it moves, as if under spring pressure, it may be a device to keep the front sight in place. In either case, you remove the sight by driving it toward the muzzle with a non marring punch in the "cross notch". If piece behind the sight does move, hold it down while driving the sight out (at least until it starts moving). Good luck.
Mike