There are actually 2 different 360mm lenses for Mamiya RB/RZ67 system, the older version is f/6.3, and the newer version is f/6, with a completely different optical design. I believe all of the RZ67 lenses are the f6 version, which I would think is likely at least a little better (otherwise why would Mamiya revise it?), but I've never seen a comparison. There aren't really any terrible lenses in the lineup, the difference between the two is likely subtle. The newest RB67 lenses will be marked K/L.