The Indus Dictionary Project
The shape depicted in Mahadevan's sign number 373 has a large and a small size in Indus script. Each size takes a different meaning. Mahadevan recognised the existence of the two different signs and created sign numbers 373 and 374.
In its smaller form, the symbol is a simple pictograph of a pack.
In its larger form, the symbol is the outline or contour of an area. In other words, the limit of area not rigidly defined by a boundary wall.
Pack: Harappa: Seal: H-479 a: Sayid Ghulam Mustafa Shah and Asko Parpola, 1991: Corpus of Indus Seals and Inscriptions: Volume 2: Page 282: Collections in Pakistan: Suomalainen Tiedeakatemia.
Pack: Mohenjo-daro: Seal: M-1958 a: Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3,1: Page 90: New material, untraced objects, and collections outside India and Pakistan: Suomalainen Tiedeakatemia.
Contour: Harappa: Graffiti on pottery rim: H-994 A: Sayid Ghulam Mustafa Shah and Asko Parpola, 1991: Corpus of Indus Seals and Inscriptions: Volume 2: Page 345: Collections in Pakistan: Suomalainen Tiedeakatemia.
In both inscriptions H-479 and M-1958, note how the word pack only takes up half the height of the character area.
Indus Script Sign Numbers 373 and 374: Sign List of the Indus Script: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.