If it was manufactured and imported between 1994 and 2004, you can't legally put a bayonet on it unless you put 10 other U.S. made parts on it. If it wasn't imported in that time period, you should be good to go. I have never heard of ATF agents checking serial numbers on SKS's in the field to determine import date. Afraid you've got it backwards there, friend. 922(r) dictates that your weapon may have no more than 10 foreign parts if it's not an importable configuration. No more than 10 foreign parts is a lot different from you must put 10 US parts on it.    Side detour: One exception to 922(r) is (for military C&R's) the original, historically relevant military configuration (hence what folks are worked up about when discussing C&R status). This is why Russian, Yugoslavian, etc SKS'es tend to have their bayonets installed and functional without breaking any laws - leaving it on is absolutely legal for a C&R weapon using its original gear.    Back to the subject: For ANY rifle, there's no Federal reason (your state's laws may vary, but not even CA cares about bayonets!) that you can't put a bayonet on a weapon if it has 10 or fewer  counted foreign parts . I've seen people at gun shows act like idiots because Cletus at the range told 'em they can just replace five screws to take care of their compliance parts. Don't be like them; 922(r) has a specific list of ... 18 or 21, I think, components which may or may not be a part of any given rifle design. Examples from the list... operating rods, gas pistons, trigger, hammer, disconnector... sling swivels and screws don't count.    Anyway, for the SKS... even if your rifle has more than 10 foreign parts, if it was imported with the bayonet it should be legal to keep the bayonet on it. However, if you were to get your parts count down to 10 or lower, you'll absolutely be out of any kind of bayonet-related Federal hot water you may care about. When it has 10 or fewer foreign parts, it is considered a domestic gun and it's not subject to questions of importability.