I've never seen a "6666" marked SKS. That doesn't mean much though.
Back in the mid seventies, I saw several ''capture'' 666 SKS rifles, brought back by a Viet Nam vet, who was a UH1B pilot.
He was quite honest about not personally capturing any of the SKS and AKs in his collection, along with a very beat up and rusty Thompson, with what he told me were French markings.
To my knowldedge, all of the "666" factory stamped SKS rifles were built for military purposes and many of them went to Viet Nam as well as some of the other nations just south of the Chinese borders.
I've never seen them in Canada but it would only be a matter of time when one or more show up.
That rifle would be worth a lot of money in the US, if its provenance can be proven to actually be a Viet Nam bring back.
Many of the firearms brought back into the US from offshore wars never were recorded or given ''trophy'' status. This might just be one of those orrrrr??????? Maybe it never went anywhere near Viet Nam or it could have been one of several thousand Century International picked up as surplus, along with a lot of other types of firearms after North and South Viet Nam were amalgamated in one nation again.