The 4" Wilkins 550 Double Check Valve backflow preventer was introduced in 1975 under the Richwell Model DC name. Approximately 1978 it was sold to the Neptune Meter Company. The Neptune Company changed the model to 550 and produced it until 1984. Wilkins bought the rights to the device in 1984 and it was discontinued in 1995. The body was made of epoxy coated cast iron and features replacable seats. Check assemblies were self contained. Hard parts have been discontinued for the Model 550 and rubber parts are the only avilable service parts. No special tools are required for servicing.