Here's what I've heard as an explanation for the for the various observed features of the rifles: They were initially designed for second line service/training in China, but the SKS came along fairly quickly afterwards and replaced it in second line service, relegating the rifles to training use only. After that, they were vastly overstocked and a large number of them were shipped to Vietnam as war aid, with the remainder being kept as trainers. The imported ones that are in horrid condition were buried for some reason (depending on who you ask, it was either: buried by China to disguise the fact that they existed, buried by the Vietcong to disguise the fact that they were receiving war aid from China, or buried by the Vietcong in weapons depots that were dug up after the war and sold back to China in exchange for Type 56 AKs), and at some point were sold by China as surplus.
One metric **** ton of them are mismatched to some degree, implying that they were rebuilt at some point. Judging by the general condition of these rebuilds, they were likely done in a hurry by a country that didn't have the time to do a complete refurb, lending credence to the idea that they were used in Vietnam. Of course, the damage is also quite possibly the result of extremely long and heavy use in a training program (see also: the Romanian "Instructie" rifles)
Purveyor of fine Mosins, Hater of Bubba, and Pantsless Evil Dictator of Eggplants.