The Nikkor 24mm f/2.8 AI-S is a lens from the late 70s but shares its optical formula with its predecesors (pre-AI and AI) and successor (AF-D). The AI-S is also quite common and as such appears on ebay frequently in various conditions.
The 24mm f/2.8 AI-S lens is reasonably simple to disassemble and teardown with the only complication being the CRC unit at the rear.
