d rail:  Actually, allowing for the difference in camera angle, the Parkhurst device pictured in Poyer's book looks very similar to your 3rd picture.  (BTW - S & S Firearms, in their 2012 catalog, had a Parkhurst clip for sale at $150)!
  You have a rare item that most of us have never had an opportunity to study.  My 'unsubstantiated hunch' is that in 1900, when the Parkhurst/Warren device was being manufactured for trial, over 1/3 of the U.S. Krags in Service were still pre-Model 1898.  I would be real surprised, if it did not occur to someone at Springfield, to try the device on a Model 1896 Krag.  (Just my opinion and hunch).  Personally, I would leave your Krag as it is;  no point in corrupting evidence.