flap seat

And be it enacted that […] no person […] presume to wish to go out for ginger-beer or oysters between the acts, to the annoyance of the nine rows of audience behind him, and the especial discomfiture of the fat old gentleman with the child on his knees, who sits on the flap-seat by the box-door.

After several minutes, Liebknecht stood barefoot in a suit of long winter underwear. Some of its buttons were missing and the flap seat was baggy from too much laundering.