If that is true then how do you explain all the mosin nagants out there that have parts that are force matched to make the serial numbers match? If what you are saying is true, there are probably illegal guns sold everyday by licensed FFL dealers to legal citizens.
NO, thats not quite right. There in only one number recognized as the serial number in common US practice (and I believe, in law), and that number must be on the action/receiver/frame of the gun. All the other numbers stamped on the other parts are nothing, from a legal point of view.
Your gun may be 12345, and have 45 or 345 stamped on half the parts. The serial # is 12345, stamped on the receiver. IT may have 45 on half the parts and 27 on two of the parts. It may have all different numbers on all the parts. This matters only to collectors. The legal serial# is still 12345...