I did a Google search for, "N. Bizen (no) ju Ichiryushi Nagamitsu saku" and it took me to the website Brian posted a link to, http://www.japaneseswordindex.com/naga.htm.
I'm curious why I only see two characters on my sword that match the ones pictured in "N" and they're the lower two on the sword unless the second one down on both the sword and "N" are the same just made slightly different like the "naga" is? Also, why are there eight characters in "N" and only four on the sword?