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 three Indus units.
Illustrative Text Reference:
Harappa: Stone Object: H-1506 A: Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3,1: Page 202: New material, untraced objects, and collections outside India and Pakistan: Suomalainen Tiedeakatemia.
Image Credit:
Indus Script Sign Number 1463: List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.