All of our horn kits follow the same format of HK-(Horn)-(Tank Size)(Compressor Model)-(Installation Bracket). The train horn kit shown in the photo above is our Conductor’s Special 244K train horn kit. The "HK" in the part number indicates that it is a horn kit. The first two letters of our part number will always tell you what kind of product it is. Other examples are "AH" for air horn ( just the horn by itself) and "HA" for HornAir (onboard air systems). The "S4" in the part number identifies the horn as our Shocker XL train horn. In our full kits, the second section of the part number will tell you what kind of horn is in the kit. Other examples are "K5LA" for the Nathan AirChime train horn and "C3B" for our Outlaw Black train horn. The "2" in 244K tells you that it is a 2-gallon kit. The "444K" identifies the compressor in the kit as the Viair 444K air compressor, with the (K) indicating that this compressor comes in a black finish (also known as our Nightmare Edition). All of our horn kits follow the same format of HK-(Horn)-(Tank Size)(Compressor Model)-(Installation Bracket). Adding accessories to the kit would also change the part number. For example, the part number for this kit with all optional accessories added on would be HK-S4-244KAEWR-STDDK. As you can see, after 244K there are additional letters that denote the accessories it comes with. "A" stands for our tire inflation kit, "E" stands for our electric drain valve kit, and "WR" stands for our wireless remote. The letters "STDDK" indicate that this kit comes with our spare tire dual delete bracket.