For purposes of federal assault weapon bans, thumbholes are fine. Some states classify them as assault features, however. And here's how it may get the SKS: Normally, things like bayonets are illegal to attach to imported rifles. Exceptions are made for C&R SKSs, but they have to be in original configuration. I think Dragunov stocks aren't included in that category. I could be wrong, and generally no one would care anyway, but it's important to be aware of.