Today in 1987, US Patent 4705188 A was issued, an invention of Eckhard F. Rahn, assigned to the Miller Brewing Company, for his “Keg Cap.” Here’s the Abstract:

A cap for covering the open end of a Barnes neck surrounding the outlet of a keg for malt beverages includes a circular, flat, resilient top; a skirt depending downwardly from the top and having an inwardly directed annular sealing bead for gripping the outside of the Barnes neck and vent means at the junction of the top and skirt. The top, annular bead and vent means cooperate to keep the cap in place when small amounts of fluid are accidentally released from the keg. The cap, in addition, to covering the outlet can bear information as to the keg contents.