The invention consists of a screw cap assembly including a manually operable handle piece rotatable in both directions, a cylindrical case mechanically coupled with the handle piece for unitary rotation therewith, a key-operable cylinder lock mounted in the cylindrical case and capable of rotating only partially and independently therefrom when key-operated from outside, and a screw plug proper operably connected with the handle piece through the cylindrical case. The plug can be rotated in both screw-advancing and screw-retracting directions when the cylinder lock is key-operated to its unlocked position but only in the screw-advancing direction when the lock is in its locked position.