The Flushmate III M-101526-F31 replacement system is designed to replace the Flushmate M-101526-F3 503 Series models. The Flushmate III M-101526-F31 1.6 GPF replacement system, is designed to replace the Flushmate M-101526-F3 1.6GPF 503 Series models for two-piece toilet fixtures. The Flushmate III M-101526-F31 includes the Flushmate M-101526-F3 complete system with locknuts and gasket to install in your toilet tank. The Flushmate III M-101526-F31 1.6GPF replacement system is designed to only work with existing pressure assist toilets that were originally equipped with the Flushmate III M-101526-F3 1.6 GPF system.