Series 775 Double Check Valve Assemblies were produced from 1998 - 2003. The smaller 1/2 - 2" devices are constructed of nickel plated copper tubing. The single cover provides quick, easy access to the two check modules which tread into the body. The larger 3 - 8" device is made of stainless steel. Much like the smaller devices, the larger size has a single cover with two check modules that thread into the body.