Watts 007 is a double check valve assembly. Production began in 1989. All versions in the 1/2"-2" size have bronze bodies with in-line modular check assemblies that are removed from a single cover on the top. Check modules are held in the body by a retainer. Check seats are replaceable. All springs are contained when the cover is removed. There have been several modifications. Repair kits are different. The original design was the 007 model produced from 1989-1991. From 1991-1993 the 007M1 model was produced. The difference being in 3/4"-1" the disc holders on the newer M1 are plastic instead of bronze and in all sizes a change in the check seat dimensions. The check cover is bolted on in the 007 and the 007M1 except the 3/4"-1" 007 which had a screw on cover. The 007M2 design began in 1993. The major change in the 007M2 was the downsizing of the body which also downsized the internal parts. In 1993 the 1/2" 007 began production. In 1998 the 3/4" 007M3 was introduced. The U007 series was developed in sizes 3/4"-2" which incorporates a union end into the body. The U007A series utilizes a 90 degree elbow for an up and down piping orientation. The 007PC series is a polymer coated bronze assembly. The SS007 series is available in sizes 1/2"-1". The SS007 has a stainless steel body assembly.