The 4" Watts 770 Double Check Assembly was produced from 1993-1997. The body is a two cover, epoxy coated ductile iron construction. The check assemblies are stainless steel toggle linkage type. They use a vulcanized clapper plate. Check springs are contained when the cover is removed. Spring tension must be removed to repair the device. The device has replacable bronze seats, but a seat removal tool is required.