Jon - I corrected some typos (missing letters) in the original text. However, I would not know how to explain it any better than I did above. Your list, your choice. Just trying to help.
The “Acorn” was factory 861 which later used the simply “8l” on headstamps. I have a Makarov round from 1960, with "81 headstamp. I, myself, have never seen a Chinese round of any caliber with the headstamp “861.” That doesn’t mean it doesn’t exist, just that I have never seen one in handling a lot of Chinese SAA.
Unfortunately, I don’t have a timeline for the changes. However, if I had to guess, I would say it was the factory at Hunan.
Maybe Shen can make it clearer, as he likely has more information than do I. That’s the best I can do.