Have seen one of those, and yes it is huge.
It was at a camera shop that used to be on BrownTrail in Bedford,TX back in the late 70s / early 80s.
i forgot what Nikon lens it was... but there was one about the time of that 6mm and 8mm when I had my Nikons, that the lens had to be mounted with the mirror locked up, because the rear lens elements went so far into the film body, to get the rear element as close to the film as possible.
I think it was the 8mm like this one
Fish-eye-NIKKOR 1:8 f=8mm No.88056
Mouse over the image to see the back of the lens without the cap to see how far into the camera body that thing went.