I see there are SWC/M models that have the newer lens casing with the rubber focusing ring with the lens designation: Prontar CF T*. There are also SWC/M models with the older lens casing (similar in look to the older SWC) with metal focusing ridges. The lens designation on those lenses are: Synchro-Compur T*. What is the difference in performance, quality and worth when obviously buying used if similar in condition? I shoot mainly B&W.