Nikkor-H 1:3.5 f=2.8cm Prototype
A number pictures exist showing prototype Nikkor-H 2.8cm lenses, with "tick marks" engraved on the focus and aperture rings.
One, probably the earliest, has lens data engraved on a ring inside the front element, so is clearly different from the production lens.
Two other pictures show lenses similar to the production version with larger front element and lens name engraved outside the filter ring.
One lens is marked in feet and another in m so at least three prototypes were made, possibly more.
Unfortunately the serial number is not known for these lenses.