Today in 1942, US Patent 2283100 A was issued, an invention of Harry C. Shores, assigned to Anheuser-Busch, for his “Pressure Regulating Attachment For Liquid Containers.” There’s no Abstract, although in the description it includes this summary:

The present invention relates to new and useful improvements in means for creating pressure within liquid containers, such as beer kegs, and the like, and has for its primary object to provide an inflatable bag adapted for attaching to a wall of the keg and projecting therein so that pressure of the liquid in the container may be increased by expanding the bag.