Welcome to the forums. Unfortunately, I can't view you pictures. However, I believe you are correct about it being Nissan manufacture. The 'N' suffix appears on all Nissan serial numbers and the prefix would place frame manufacture at the end of 1972. That's not too far off the component codes.

Based on the evidence, it's too early to be a Shogun. Known brands that Nissan manufactured include Corsaro, Kabuki and Matsuri. Kabuki head badges did use the face of Samurai but typically included the Kabuki brand name.