The Indus Dictionary Project
This is the noun stream with a point of light at the east cardinal point, and points of darkness at each of the other cardinal points. Hence this symbol makes specific reference to a waterway located to the east.
Harappa: Incised Tablet: H-325 A (NB. image H-325 A is the reverse of Mahadevan's sign number 1162): Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 226: Collections in India: Suomalainen Tiedeakatemia.
There is only one extant example of this Indus sign.
Mahadevan was quite right to assume that tablet H-325 was intended to be used to create an impression. Otherwise, the stream would have the wrong orientation.
Note the alignment, in Mahadevan's sign number 1162, of the point of light at east with the left-hand side of the point of darkness at north. The position was chosen to avoid confusion with the symbol for dusk.
Indus Script Sign Number 1162: List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.