The RP050 from ISC is now a Hot Forged aluminum block made from high tech aluminum alloy.
It is incredibly light, but strong. Aluminum alloy bushings and sheaves are supported by stainless steel axles.
Spring lock captive bushings are easily accessed by a quick-twist nob. Sheave is supported by a sintered bronze bushing.
Side plates are broad enough to protect lines during use and painted with a powder coated finish.