Today in 1988, US Patent 4779750 A was issued, an invention of Kenneth J. Armstrong, for his “Beer Keg Cap.” Here’s the Abstract:

A removable thermoplastic cap for a beer keg bushing comprising a flat disc with integral peripheral skirt which includes inturned upwardly directed gripping lugs preventing removal of the cap when once fitted to a beer keg bushing, vertical slots spaced around the skirt and extending partway across the disc to provide venting for the interior of the bushing, and a pull tab provided on one side of the disc and connected by weakened lines to the opposite side thereof to facilitate peeling off of the cap by tearing apart the skirt.