Ronstan Series 50 All Purpose Block - Double. 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 50 All Purpose Block: Manufacturer: Ronstan Color: Black Package Contents: Ronstan Series 50 All Purpose Block Ronstan RF51200: Series 50 All Purpose Block - Double