The stand pipes feed the HPO to your HPO rails so they can in turn feed the injectors. In 2004 ford designed the HPO rails so that they can be interchangeable to either side, whereas the 2003 were HPO logs and were only available for one side only. Since the stand pipes are in the rear of the rails than the dummy plugs go in the front plugging the hole in the HPO rail that would potentially be the stand pipe hole if you put that rail on the other head of the engine.
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