These ferrules are meant to protect drilled holes on a pull buoy that were made for rubber straps to attach the buoy to the thigh.

This is useful, for example, in swim run, so you can swim with it, run with it and not worry about it losing it.

The straps are tied to a loop around the buoy. Total of 8 ferrules were used (4 holes, both sides), and the ferrules are glued to the pull buoy.