I noticed this on both CCTV and "TV lens" C-mount lenses.
Is this for manual black level calibration, or protecting some early sensor or *icon tube style from wear, or entirely another reason?
It does give interesting photographic results when stopped down to a hair's breadth, you get visible spheres from all the dust in the system :)
Also, some "TV" lenses have a square vignette inside that seems to be only effective if the lens is wide open - if this helps with stray light or similar problems, why has this not been implemented in still camera lenses - while you risk the vignette being misoriented, modern (post-screwmount) camera mounts do force an orientation...
(while these are primarily videography lenses, C mount lenses can be used for still photography too, so I did not consider this completely OT).