Questions and discrepancies regarding the Nishiki Data Base / frame encoding.

I have pictures of a late 1970's Nishiki Professional where the frame code starts with the letter N.
See the following picture.*

*If I remember correctly, this Olympic with its pictures was presented here in the forum some time ago.


I can not explain this N myself with my available documents. There is no known or planned N for country/import coding.

Who can provide clarification on this?

Thanks for any informations .