This is a pictograph of a doorway set into a stockade fence. Hence, it is the noun stockade entrance.
Illustrative Text Reference:
Harappa: Seal: Find number 995, Mound F: Madho Sarup Vats, 1940: Excavations at Harappa: Being an Account of the Excavations at Harappa carried out between the Years 1920-21 and 1933-34: Volume
II: Plate XCVI: Tiny Seals: Image 451: Government of India Press, Calcutta.
This is Mahadevan's sign number 4451, which I have redrawn to better reflect the original.