Perusing the Chicago sectional, I ran across a surprising notation within the E-G airspace boundary transition around Platteville (PVB). Foreflight to the rescue. Switching Maps view from "Sectional" to "Aerial", and zooming in, revealed the truth: there really is a "M" constructed at ground level. No idea why, or if it's meaningful to a pilot searching for Platteville airport. But there it is.