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
Rare Lens] $160,000 Nikkor 6mm 2.8 Fisheye Can See Behind You | Fstoppers
The 1972 Nikon 6mm f/2.8 fisheye - 9GAG
Camera Geekery: The Nikkor 6mm f/5.6 Fisheye Nikkor aka The Holy Grail - Japan Camera Hunter
Nikkor 6mm f/2.8 fisheye lens video and photo samples - Nikon Rumors