Virtually all of the Russian rifles I've handled, own, and seen have had a shellac finish. It was their finish of choice. It's cheap, easy to apply, maintain and repair.
I've seem some very early SKS rifles and their finish was original to the stock in my very humble opinion. It was also a deep red/brown. Some of that color has been picked up over the many decades of handling, debris being embedded and UV. The original shellac they used was simply a non-filtered shellac that was amply available at the time. Shellac, in it's "raw" state is a deep red for the most part. It's really just "bug spit" that is collected from trees in places like Thailand. Today it's filtered to provide colors from deep red to water clear.
You may have seen some of the Albanian stocks in the recent import of those SKS rifles. Their stocks are also shellac but it looks like they had monkeys using old camel tails to apply it. It's also a very clear version of shellac,too, which makes the stocks very blonde. It's easily fixed to be more attractive.
It's curious that the Finn Russian rifles had their stocks refinished but not surprising, I guess. They had their own stock finish process just like the Russians or the US arsenals. So, when they refurbed them, they simply stripped the stocks and applied their finish. But it is curious that all of the the SVT40 Finn captures I've seen still maintain their shellac finishes.....at least that's what's on mine. Maybe the Finns just decided they didn't have time to strip and refinish that rifle during the very tight war-time footing. When allowed more time, however, they did refinish other rifles. It's just speculation on my part.
If you want to double check as to what's on your stock, simply take a rag with a little denatured alcohol on it and wipe a small area like the bottom of the buttstock or some other inconspicuous place. You won't harm it. Give the wet spot a few seconds and then see if it will "tack up". It will if it's shellac.
Shellac finishes can be fairly easily repaired by using new shellac either purchased in the same color or dyed up a little to add the brown/red color. That allows you to retain the original finish but firm it up so it continues to protect the lumber for decades to come without harming the originality of the stock. Soldiers were required to maintain their rifles and now that we own them, it's up to us to continue to do so.
Rome