"What is this hinged flap at the bottom of each section of screened porch on a 120-year-old house in Georgia (USA)?"
A: "Covered scuppers or in this case, sweepouts. The covers allow you to open them inward and upward with your foot, then sweep out dirt/water. Gravity or a spring closes sweepout door after you move your foot so no critters (mosquitoes, small bears etc.) get in."