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This Tow Ready hitch pin and clip set is sized for 5/8" receiver holes and is what actually locks your ball mount, cargo carrier, or other receiver-mounted accessory in place once it's slid into the tube.
The zinc plating gives it a layer of corrosion resistance for life outdoors, and the clip keeps the pin from working loose over rough roads.
A small part, but not one you want to be without — good insurance against losing your ball mount pin at the worst possible time.