Most of the ones not SA marked would be US manufactured ones that were never sent to Russia (I have one made by Westinghouse, no SA marks, tons of US arsenal marks) Many did get sent, of course, so they either ended up in Russian hands and rebuilt or in Finnish hands with all the different things they did to them. So, Dimon, the one you have is likely from the batch that did make it to Russia - debatably it as a richer history than one that mostly sat in an arsenal in the US, maybe getting pulled out for a bit of reserve service during WW2.
Other manufacturers - exceedingly rare. I have a russian made one with no SA marks but in all honesty I think it's a case of the SA mark just being missed. (It's a 1915 Sestroretsk with a German stamp on the stock, no finnish stamps but has a suspiciously finnish looking sling hanger)