This is the symbol for area with the addition of a line which represents a third dimension. Hence, it is an ideograph for a cuboid. A cuboid was a measure of capacity. It can be likened to the Mesopotamian gur. In this instance, the cuboid is divided into five Indus units.
Illustrative Text Reference:
Harappa: Seal: H-558 a: Sayid Ghulam Mustafa Shah and Asko Parpola, 1991: Corpus of Indus Seals and Inscriptions: Volume 2: Page 292: Collections in Pakistan: Suomalainen Tiedeakatemia.
Image Credit:
Indus Script Sign Number 4220: List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.