Marc Batters , Jul 06, 2010; 03:44 a.m.
I am familiar with the RZ's and not the RB's from Mamiya. Having said that, which series of camera body do you have in the RB line:
Pro, Pro-S, or Pro-SD?
My thoughts are, that the KL lenses have the larger throat diameter of the RB Pro-SD, (same as the entire RZ family), so I'm not sure that you can mount a KL lens on a non "SD" RB body.
Also the KL series apparently have better multi-coated elements, much like the RZ lenses. Hopefully someone with more RB experience will chime-in addressing shutters in the RB family of lenses. I seem to recall that at some point, the shutters were upgraded/improved, but I can't recall the time frame in the series, or more specifically, which lenses may have been affected.
Your wonder that the older C series might be the equal, or better than the KL series, (basing this on comparable pricing),
may just mean there are more RB Pro, and Pro-S camera owners looking for lenses, than Pro-SD owners.
The Pro-SD was produced simultaneously with the introductory run of the first RZ Pro models, so there may have been far fewer RB Pro-SD camera bodies produced than RB Pro, and Pro-S models.