This is the noun street corner, to which the scribe has added an aerial view of two walls. It is an ideograph for a walled corner pitch. The size of the pitch in Indus units is given by the number of bricks in the wall. Hence, this corner pitch has the dimensions four Indus units by four Indus units.
Illustrative Text Reference:
Mohenjo-daro: Seal: M-389 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 95: Collections in India: Suomalainen Tiedeakatemia.
Image Credit:
Indus Script Sign Number 2157 (Street Corner Pitch 4 by 4): List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.