Round endcap which slides into a round tube.  The endcap is smooth which means that the Inner Dimension (ID) of tube and Outer Dimension(OD) of the endcap sleeve must match.  The Outer Dimension (OD) of this endcap sleeve is

The Outer Dimension (OD) of this endcap flange is .75" (19.05MM) which should be used to match up the size of the tube you are looking to cap.