Round endcap which slides into a round tube.  The endcap is Multi-Gauge and ribbed which means that it will fit in several sizes of Inner Dimension (ID) of tubes and holds fast once inserted because it colapses when inserted.

The Outer Dimension (OD) of this endcap is 1.75" (44.45MM) which should be used to match up the size of the tube you are looking to cap.