This is a pictograph of a stream with a point of light at the south 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 south.
Illustrative Text Reference:
Mohenjo-daro: Seal: M-942 a: Sayid Ghulam Mustafa Shah and Asko Parpola, 1991: Corpus of Indus Seals and Inscriptions: Volume 2: Page 91: Collections in Pakistan: Suomalainen Tiedeakatemia.
Comment:
Note the alignment, in this Indus sign, of the point of light at south with the left-hand side of the point of darkness at west. The position may have been chosen to avoid confusion with the symbol for dusk.
Image Credits:
Southern Waterway: Lynn Fawcett, 2020.
Indus Script Sign Number 103: Sign List of the Indus Script: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.