From xtremediesel.com:
Attention 2004 Model Year Owners. to ensure that you have an early 2004 Dodge Cummins 5.9L Diesel you must check the engine tag (A Red or Silver Tag) located on the drivers side of your engine. The tag should show a horsepower rating of 315. If it does not you most likely have a 2004.5 Dodge Cummins 5.9L Diesel.
Attention 2004 Model Year Owners, to ensure that you have a 2004.5 Dodge Cummins "600" 5.9L Diesel you must check the engine tag (A Red or Silver Tag) located on the drivers side of your engine. The tag should show a torque rating of 600. If it does not you most likely have a 2004 Dodge Cummins 5.9L Diesel.