The 4" Watts 957 Reduced Pressure Zone (RPZ) began production in 2000 under the Hunter name. In 2002, Watts bought the rights off of Hunter and is still manufacturing the devices. The body is constructed of stainless steel. The 957 Series features a light weight design and short lay length. It is also offered in a number of configurations with many valve options. To access the check assemblies there is a stainless steel sleeve that will slide open. You must unthread the #3 test cock then slide the cover. The 957 uses two self contained cam checks that use a tri-link check mechanism. Numerous repair parts are available for the 957 series devices, including rubber parts and complete parts. The relief valve can be unthreaded from the body for ease of service. The 957 uses an extrnal relief valve sensing line. There are no special tools required for service.