By the Late 1960s the army found that GIs were not securing the hood with the hold open device and through movement and or wind the hoods where slaming down on top of soldiers causing injury and in some cases death. A duce and a half hood will cause some serious hurt on your noggin. Anyway it started to become part of the vehicle markings to put the hood warning on the inside of the hood. The CP-73 I'm not sure about. I've seen things like OCT-73 or 10-72 being the month and date the antifreeze was changed. Maybe its Changed Prestone 73. If that's your truck Bob it's time to change the antifreeze...
I found a bit more info on the underhood lettering in question. Per the Standard Catalog of US Military Vehicles, 2nd Edition, it states that the hood lettering started being applied on Dec 29, 1955 per AR 746-2300-1 using white paint with 1" letters. There's no mention of the "CP" code.