M645
IIRC All Mamiya M645 lenses have "Mamiya Sekor C" on them, but:
1. C versions (no other letters) - oldest M645, smooth, all-metal construction, lower contrast (single coating?) bit very nice tonality, can be more prone to flare. My pref.
2. S versions - Have same lettering as #1 but also an "S". Transitional model. C construction, but N-style multi-coatings. More contrast, somewhat less prone to flare.
3.N Version. "N" lettering as well as #1. Most modern M6456 prior to AF. Plastic/metal construction. Lighter. S-style coatings (or vice-versa). MAY be less smooth in operation if older.
Resolution between versions the same, just matter of global contrast, flare, weight and construction.