AFAIK, that N model was non-US only. I'm pretty sure the N never came to the US.
The M model pictured was not known here as a CB400M, but rather, a CM400. And, there were several versions over the model years - CM400E was an 'economy' model with spoke wheels and front drum brake; CM400C was the 'custom' model as shown - Comstar wheels and front disk brake; and even a semi-automatic model - CM400A.
The CB400T shown was also known as the 'Hawk'.
In the last year or two, the displacement was bumped from 400cc to 450cc, but styling pretty much stayed the same except for the CB450SC Nighthawk model wearing slightly different cruiser clothes.
I'm really partial to the 80 ~ 83 CB400T/450T version. Styling looks very similar to the bigger 750F & 900F models of that time. If one came up for sale near me, I'd probably wind up buying it...
Be sure to post some pictures of your OWN bike if you get a chance.
Kirk