kaziboku:  Your Krags definitely appear to be cut-down rifles.  Since the originality is gone, it makes them great "guilt-free" shooters.  None of your posted pictures show a clear view of the receiver markings, so it is hard to give feedback or information.  I have never encountered a U.S. Krag without at least a damaged serial #.  But, I suppose anything is possible in a mass produced item. The serial number marking is normally about the same depth as the "U.S." and Springfield & Model Markings.  Given the surface hardness of the Krag action, it is unlikely the serial number would be removed by excessive polishing.  It would require grinding, machining, or welding to remove all evidence of a serial number.  In my opinion, discoloration and disruption in the metal surface of the receiver should be noticeable.  A clear side view picture would be helpful.