Anyone know what the original finish was on the sks stocks?

Yugo's and Russian?

The only "finish" on my unissued Yugo was the cosmolene!
But the wood was so nice that I just rubbed it down with Ballistol after cleaning it. I like it just fine that way.

What is Ballistal?

Ballistol (Short for Ballistic oil.) Originally developed in France for the Imperial German Army.
Its an oil used for guns, leather conditioning and wood preservative. Has anti bacterial properties too. (No mildew in your leather goods!) Doesn't break down and form acids or goo as it ages. Very good all purpose oil. I've used it for years.
You can even mix it with water and it forms an emulsion. Spray down a motorcycle before you store it to keep rust away! The water evaporates leaving a light film of the oil in place.
Has many cool uses.