From researching for my book series, "the AK47 catalog", the Chinese had over 20 different trunnion factory codes.. 66 being the "main" and most common one.
"The following codes have been observed by various sources; 26, 36, 56,66, 68, 116, 266, 313, 356, 386, 416, 626, 962, 976, 5506, 9236, 9336, 9616,
9696, and 9846. This does not count rifles that only have a serial number.
Many of the factory codes seem to be a play on the original factory number in
order to hide its true origin.
The are subtle differences between many of the rifles produced at the
different factories, but between the factories possibly having multiple codes,
not all factories producing rifles at once, many of the factories only assembling
rifles using parts made by other factories, and factories sometimes using old
parts, it difficult to pinpoint if the differences were an intentional design
change, or natural variation from everything being made manually."
Other then the early "slabside" rifles, it's difficult to try to date a Chinese rifle. There have been "collector theories", but they seem to have been busted.
From researching though, the original Code 66 factory that was built by/with the Soviets is closed and long empty.