This is a bulk package of 10 stainless steel magnetic nested endcaps. The inside diameter is 8mm. It is 20mm long and 12mm at its widest point.
This is a nesting endcap, which means one portion of the endcap slides inside the rest of the endcap and is completely hidden from view when closed.
When gluing it is best to keep the two pieces separate to ensure that they are not glued together.