Today in 1971, US Patent 3618632 A was issued, an invention of Frederick F. Stevens, assigned to Hoff Stevens, for his “Check Valve.” Here’s the Abstract:

A fluid check valve having a hollow elongated self-supporting nipple supported in a path of fluid flow and made from a resilient deformable material. A slit through the closed upstream end of the nipple divides it into adjacent sections which are laterally displaced from each other to provide a path of fluid flow there between when the fluid pressure on the downstream side of the nipple exceeds the fluid pressure on the upstream side thereof. Under the influence of back pressure the sections resiliently press against each other to prevent reverse fluid flow there between. The nipple has an integral part which supports it in a fixed position in an associated conduit member and also forms a seal between the latter member and another conduit member connected thereto. A rigid nipple insert may be provided to prevent inward collapse or deformation of the nipple due to fluid back pressure.