The manufacturing coding on these is not something I have listed. J-NT, J, or NT.
I was hoping that the NT was a US company and that the J in front was a designation for it being for export to Japan.
So even on the South Korean KM ordnance that I have seen, they have "US" designations, with the K in front as Jeff stated. Example: M15 to KM15
However, they still use their factory codes since they are manufactured under license in South Korea.
So, by that example if these are Japanese and made under license, I would guess that the codes would be Japanese.
Again, I could be wrong, and I don't have a list of all the codes out there, even US ones.
Joe