Abstract: A flange for a container opening has an internally threaded aperture to receive a plug, and a plurality of external serrations in a circularly spaced-apart relationship around the periphery of the upper portion of the flange. The plug is externally threaded and, when mated with the flange, closes the aperture. Both the flange and the plug are molded out of plastic material as single integral components. On the lower portion of the flange is an annular lip which is positioned beneath and extends radially beyond the external serrations. A container suitable for the flange has a boss around the container opening in a wall (usually the end wall) of the container. The boss is formed with internal serrations equal in number to the number of external serrations on the flange. This boss can be crimped onto the flange placed in the opening, such that the two groups of serrations permanently interlock with one another thereby securely locking the flange into the container.