I have a theory, thats pretty well un-provable without having every sks ever in front of me, or a big bunch of SKS nerds on the internet.
It relates to stocks, and renumbered repaired stocks in particular.
When i think about the minor repairs on some of my stocks i cant help but think how it would be unlikely for # A1234 that needs a stock repair to sit around on a table while a skilled carpenter inspects it, cuts the old wood out at the best spot, cuts a new piece to replace it, test fits it, sands it, tests it again, eventually glues it and clamps it overnight, then slaps a little varnish or whatever on it the next day and lets that dry.
My theory is, in the above scenario the stock went in the "to be fixed" pile and one from the "ready to go" pile was put on. Either a new or a repaired one depending on a million reasons.
The problem is to try and prove this you need to find gun #A1234 and then a stock with #A1234 xx'd out. And who the heck is that nerdy they would keep track of that kind of thing??