So far as I'm aware, the optical formula (7 elements in 5 groups) of Nikkor S 50/1.4 remained the same until the "K" version.
Correct. The thick dark line indicates a new optical design. Medium dark line represents a minor adjustment of the previous optic (change to glass material, curvature and/or spacing of lens elements). The medium light line is an upgraded barrel design with the same optics, and the hairline is a new serial number range.
The original 7-element 5-group Nikkor-S 50/1.4 has three variants:
- original Nikkor-S serial no 314111 - 454670.
- upgraded Nikkor-S from 465011 - 1613735. This includes the multicoated Nikkor-S.C version.
- Further upgrade for "New Nikkor" (K) 50/1.4 from 2797021 - 3004000, probably to improve performance at close range since it focuses closer.
After these lenses an all-new design was introduced with 7 elements in 6 groups, starting with the new K version (pre-AI), through AI, AI-S and AF versions.