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I think the smaller pipe goes straight through without the resonating affect and the bigger pipe goes through WITH the resonating affect? Like the bigger pipe passes through the part with the holes that go through the fiber glass and the smaller one is just solid. Thats my guess.
From the looks of it "tuner cap", you could plug one of the holes to produce different sounds. Plug the smaller hole for deep loud ear bleeding and the big hole for the quiet rumble. That is my take on it.