Breakdown of RB lens series
To the OP: The lenses were made in three series:
Original series (Mamiya-Sekor)
All lenses have chromed noses, and nice thick knurling on the lens locking ring on the rear. All of the are single-coated (amber/yellow reflections off lens) as per 1970 technology. Nothing to write home about, though - as with any decently-designed lenses of any area for MF cameras, can yield excellent image quality. More prone to flare, etc.
C-Series (Mamiya-Sekor C)
Green-colourd "C" suffix. All lenses except the fisheye have chromed noses. Finer beveled knurling on lens locking ring on rear. All of the lenses feature multicoating (purple/blue/green reflections off lens) - and what an amazing multicoating this was. You'll go blind trying to make most of these flare. Greatly improved performance. Top level of build quality for RB lenses, with best size/performance ratio also.
K/L-Series (Mamiya-Sekor K/L [L-A])
Quite uncommon on the mechanical RB system, as these lens formulations are more commonly used with electronic shutters on the RZ system. These represent the top level of performance. None feature the silver ring on the front. These lenses are really massive (particularly the 65 and the 140 Macro are a couple of centimeters bigger than their C-series predecessors, but it seems "wasted" space, i.e. the glass itself is not much bigger. Uses more modern materials (e.g. some plastic on the shutter sped ring, etc) and generally not as nice-feeling as the C-series, IMHO.
There are a couple of unusual, faster K/L lenses, like f/3.5 lenses in 75mm and 150mm. I myself prefer the C-series lenses on my RB - I compared one or two, but could not, for my purposes, see any difference, and the C-series lenses are smaller, prettier, and cheaper. If you use a digital back, though, I'd suspect you'd want all-K/L.
