Oversized Vents

We have had several orders recently for our M sized mondo units for old homes with cast iron 4 inch vents. This is the actual diamter of the vent, it has a flashing that is about 5 inches in diameter. So if you have a home built prior to 1960, check this before ordering. If your vent is cast iron, odds are its an old vent and may require the M size unit. But calll first so we can confirm