Since there are no matching serialed ones to guns, why on earth would anyone go to the trouble to serial them?
Seems plausible that some were made with them, perhaps they thought they flat out sucked, pulled the installed ones, and mothballed them with the rest for potential aid or maybe for refurbishment purposes?
One might think if no matching ones have been seen that there aren't any shenanigans going on...just a few that made it to production guns, only to be discarded later on, and occasionally popping up as a replacement stock.
I find the theory they were for the Vietnam conflict a bit thin...since they are not seen on bringbacks. I lean more towards them being another of the Chinese 'improvements' that didn't pan out and was later dropped. Perhaps a wood shortage or other event that caused them to explore other options temporarily? It's possible they made a bazillion of them before realizing they didn't want to go that direction...
I also don't seem to recall seeing any bringbacks with the spring slings either...also purportedly for the Vietnam jungle rot issues... I can see them both as afterthought redesigns being more plausible.
I find it fortunate (for me) that they are so ugly that I am not really ever tempted to get one...especially at today's prices, usually sans handguard, and quite often missing essential impossible to find hardware. If I ever located a serialed one, I might bite...I don't have a gun I would be willing to put it on at the moment though. I do have a spare slot in the orphan stock racks.