The receiver pin is Quality S's standard receiver pin. It is made from 5/8" round solid steel bar stock and is plated for long life. The included spring clip keeps the receiver pin from vibrating out of place under heavy usage. Remember, receiver pins do wear out over time and it is a good idea to replace them when they begin to show signs of wear. The MRP is Quality S's 1/2" hitch pin for the mini receivers. It is made from 1/2" round solid steel bar stock and is plated for long life. A spring clip is included. The LRP is the Quality S 5/8" receiver pin with a larger hole drilled in the end. Through this hole the included padlock is passed. When in use, the padlock does double duty taking the place of the spring clip and providing theft protection. HLK is the standard hitch pin with the strength of a machined lock added on. DLock is the ultimate hitch lock. Machined from solid steel, this deadbolt style lock with its keyed locking mechanism will provide years of security and good looks for your hitch. HC33 is the ultimate hitch lock times two! This set is the 5/8" and 1/2" sized DLock packaged together and keyed alike. Machined from solid steel and plated to give years of security and good looks.