Today in 1981, US Patent 4291821 A was issued, an invention of James E. Nezworski, assigned to The Perlick Company, Inc., for his “Keg Tapping System Unit and Valve Interlock.” Here’s the Abstract:

In a keg tapping system, a keg unit in each keg has coaxial gas and liquid valves, and a tavern unit, detachably connectable to the keg unit, has a lever actuated plunger that is depressed to open the valves in the keg unit and to open a gas valve in the tavern unit. The keg unit has a well in its top in which the tavern unit is rotatably received and diametrically opposite lugs projecting into that well to cooperate with a flange on the tavern unit body in providing a bayonet connection between the units. Affixed to the lever on the tavern unit is an abutment carrier having abutment portions cooperable with said lugs. If the bayonet connection is not fully engaged, one abutment portion engages an upper surface on one of the lugs to prevent the lever from being swung down to its valve-open position; with the lever down, an abutment portion engages a lug to prevent rotation of the tavern unit to a position at which the bayonet connection is disengaged.