Plafond: "A ceiling formed by the underside of a floor, esp. when of elaborate design." [Webster's New International Dictionary, second edition. 1946]
This ceiling over the forestage is formed by the floor of what Henry Warren called the "Picture Room," a room opening off the passage over the proscenium backstage. This room is seen clearly in the 3D model drawings of the carpeneter's shop and the passage over the proscenium.