This is a pictograph of matting of the kind used on carts with a hollow frame chassis. The grid depicts the woven nature of the matting, and also gives the dimensions of the matting, which was sized to fit the cart chassis. In this instance, the matting is three indus units wide by four indus units long.
Illustrative Text Reference:
Harappa: Seal: H-72 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 185: Collections in India: Suomalainen Tiedeakatemia.
Image Credit:
Indus Script Sign Number 2431: List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.