I know very little about Chinese SKS. Trying to learn more. Questions to forum members:
I'm assuming you're limiting this discussion to /26\ guns only? Add other arsenals and everything is out the window. Even limiting to only /26\ there are oddities and everyone may not agree.
(1) When did Chinese eliminate lightening cut on bolt carrier, end of 10 m or early 11 m?
Somewhere between 11,107,000 and 11,135,000
(2) When did Chinese eliminate the groove on up side of rear sight leaf, end of 10 m or early 11 m?
Somewhere between 11,000,600 and 11,026,500 though there are instances of grooves all the way to S/N 11,107,000. Could have been old stock being found and used or a replacement part.
(3) When did Chinese change safety level from 'checked' to 'smooth'?
I have a single year 9 example that is smooth, but it could easily be a replacement. All year 10 examples appears to have knurling. First definitive non-knurled safety lever I find is around 11,300,000.
(4) When did Chinese change the groove on down side of rear sight leaf from 'narrow' (like Russians) to 'wide'?
Unknown, area is almost never photographed. In fact, I only have 4 photos:
That year 8 example is a pristine public security gun with original stock. I have no reason to believe that sight was replaced based on the rest of the gun. But again with only 4 data points, impossible to make any kind of definitive determination when this changed.