Today in 1974, US Patent 3848631 A was issued, an invention of Merton R. Fallon, assigned to Draft Systems, Inc., for his “Beer Keg Protective Device.” Here’s the Abstract:

The safety device is disposable in the gas line connecting between a gas source under pressure and a tapping device installed in a keg. The safety device includes a valve housing carrying a spring biased axially movable piston including a stem and an axially adjustable element on the upstream side of the stem, the element and piston having axial passages. On the downstream side of the piston, a slit valve is provided. In use, gas flowing into the valve through the passages in the element and piston flows into the keg through the slit valve. Upon an increase in gas line pressure above a predetermined pressure, the gas acts on the piston to displace it in an upstream direction against the bias of the spring to seal the stem against the element and preclude further ingress of gas into the keg. When keg pressure is relived as by drawing beer from the keg, the spring returns the piston to open the gas passage through the valve at the predetermined pressure. The gas check valve precludes backflow of liquid and gas through the gas passage.