This is the contour of an area to which the noun border has been added. Hence, Mahadevan's sign number 4266 is the noun border region or border area.
Illustrative Text References:
Harappa: Seal: H-130 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 197: Collections in India: Suomalainen Tiedeakatemia.
Harappa: Seal: H-642 a: Sayid Ghulam Mustafa Shah and Asko Parpola, 1991: Corpus of Indus Seals and Inscriptions: Volume 2: Page 305: Collections in Pakistan: Suomalainen Tiedeakatemia.
Image Credit:
Indus Script Sign Number 4266: List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.