Hoya a répondu à ma question - en partie... si ça peut aider :
N filters are a discontinued model.
Regarding the difference in (O) and (C) it's used to show in which way the UV property is added to the filters: UV (C) filters have a UV (C)oating while UV (O) get their UV cutting property by the use of special (O)ptical glass.
Of the 2 types UV(O) have the better quality.
Best regards,
HOYA supporting unit, Kenko Tokina Co., Ltd