The shape depicted in Mahadevan's sign number 5090 has two versions, a large and a small size, in Indus script. Each size takes a different meaning.
The symbol can be best understood by reference to Xu Shen's etymology for the Chinese character wéi 囗. Wéi is defined as making a circuit around a body of land. He may have been thinking of the practice of walking the boundaries.
Hence, the square is an ideograph for the perimeter of an area of land. In its larger form, the symbol can be translated as the noun or the adjective land. In its smaller form, it represents a boundary marker or a boundary stone for that land.
Illustrative Text References:
Area: Harappa: Seal: H-682 a: Sayid Ghulam Mustafa Shah and Asko Parpola, 1991: Corpus of Indus Seals and Inscriptions: Volume 2: Page 311: Collections in Pakistan: Suomalainen Tiedeakatemia.
Area: Harappa: Tablet in bas-relief: H-1981 A: Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3,1: Page 271: New material, untraced objects, and collections outside India and Pakistan: Suomalainen Tiedeakatemia.
Marker Stone: Harappa: Seal: H-1077 a: Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3,1: Page 148: New material, untraced objects, and collections outside India and Pakistan: Suomalainen Tiedeakatemia.
Comment:
It is worth mentioning here that a square can be divided into two rectangles or mat shapes. The mat is another Indus symbol. This brings to mind the tsubo, which is an archaic Japanese unit of measurement for area. The square tsubo is the base unit for other units of measurement.
Image Credit:
Indus Script Sign Number 5090: List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.