Metal Wingtip Skid Insert
Metal wingtip insert.  Sold 1 at a time, not in pairs.  Screw into the Wingtip-Skids shown above.  Designed to keep the plastic of the skid from wearing away rapidly on hard surfaced runways.  Included with the Wingtip Skids.  Order extras here. 

The hole in the tip of the insert is square shaped.  If you don't have the correct tool, a right-sized screwdriver will do the job.