The difference between Nagamaki and Nakamaki is that Nagamaki originally had a longer nakago (end of the blade) while Nakatsumaki were lengthened by extending the nakago of a long Japanese sword of regular length.
違いは、最初から茎を長く作ってあるのが長巻で、通常の茎の長さの大太刀の柄を延長して長くしたものが中巻とされる。