Not all military guns will have numbered stocks. Many of the early factory 26 guns with blade bayonets were imported to the US/Canadaicon in the 1990's and a good many of those were reconditioned somewhat in the PRC before coming over. Worn/damaged stocks were sometimes replaced with new production wood (no serial number and blonder than typical military stocks) and items like the gas tube were replaced if the originals were badly pitted, etc. Often the guns were also reblued. Think of it as arsenal refurbishment as this work was all done in military factories.

Also, what we think of as "commercial production" is just guns assembled from military production spare parts for sale in the states. The Chinese had stopped SKS production at a number of factories and their vast stocks of replacement parts were hastily assembled in a variety of commercial configurations for the civilian market so that some use cold be made of these orphaned spares.