Ronstan Series 40 All Purpose Block - Single Cheek. Self-lubricating Acetal polymer sheaves running on polished stainless steel races perform equally well with dynamic loads and high static loads. For durability and a long service life these blocks are the right choice for sheets halyards and control line applications for cruising and racing.Captive Lock universal head can be fixed in either of two planes or free to swivel as required. Specifications for Ronstan Series 40 All Purpose Block: Manufacturer: Ronstan Color: Black Package Contents: Ronstan Series 40 All Purpose Block Ronstan RF41811: Series 40 All Purpose Block - Single Cheek