Lensfinder.com - Ultra Rare Nikkor 6mm f/5.6. Only a few hundred of these lenses exist. This lens has a 220 degree field of view for Full Frame 24x36mm sensors. Made in Japan
Nikon's 6mm f/2.8 Fisheye on a Z 9 - first time ever? - YouTube
Nikkor 6mm f/2.8 Fisheye Nikkor lens
Camera Geekery: The Nikkor 6mm f/5.6 Fisheye Nikkor aka The Holy Grail - Japan Camera Hunter