Why is there different lettering and numbers on the spark plugs from one side of the engine to the other. They are close but slightly different. They are the original plugs. Thanks.

 

not sure what truck you have. but on some older unit's. half of the plug's fire backward's. so they put the platnium on the oposite side of the plug. depending on the way it fire's. when you replace them. they all should have it on both side's of the plug's. sound's funny i know. but that is the way it is. i think it save a few penny's when built.