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My comment about the grommet size differences was not focused on it's ID but the OD/shape. For a mechanical cable and casing to fit through the frame's hole and do so at a "working" angle means a somewhat oval hole (in the frame) is usually needed. The cable/casing won't bend without binding if the hole is only as large as the casing is in diameter. But a electric wire can bend rather tightly and be able to enter the frame at about a 90* angle with no function suffering.
So a frame made for either mechanical or electric shifting likely has an oval port. Frames made specific for e shifting only need a round hole. The electrical system grommets are made in a couple of shapes to allow for this choice. Andy (who is waiting for the side bar of whether designing a frame for only one type of option is a best choice)
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AndrewRStewart