CESSNA NACA PLUGS & OIL COOLER PLUGS
NACA inlets are air intakes that are streamlined into the fuselage. They're named for the National Advisory Committee to Aeronautics (NACA), the forerunner of NASA. Some are used for fresh air inlets, engine cooling, some as ram air inlets.
The beauty of these inlets is that they scoop air with very little drag, much better than the typical scoop that protrudes into the airstream.
NACA found out that the specific aircraft like shape is ideal to slow down air at that specific point and moisture will not freeze. So the duct will always stay free of ice.