You are looking at a feul rail delete plug For 2003-2007.5 5.9L Cummins.
These plugs are very important to have installed if you are running a tuner that is turning up your rail pressure. Because many times with tuning that goes over factory spec you can cause the factory rail pressure relief valve to blow which can crack injector nozzles and bodies.
For more info:406-728-3939 or 406-543-3900 Isaac/Ryan
For best serves please call us phone#406-543-3900 or 406-728-3939 Ryan/Isaac
Picture below is NOT actual but very close to what you are buying