The Indus Dictionary Project
This is a glyph for the beginning of an area, which is surrounded by three points of darkness and one point of light. The point of light is situated in the cardinal direction east. Hence, this symbol makes specific reference to an area located to the east.
Mohenjo-daro: Seal: M-1721 a: Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3,1: Page 29: New material, untraced objects, and collections outside India and Pakistan: Suomalainen Tiedeakatemia.
This is Mahadevan's sign number 1074, which I have redrawn to better reflect the original.
Note the alignment, in this Indus sign, of the point of light at east with the left-hand side of the point of darkness at north. The position may have been chosen to avoid confusion with the symbol for dusk.
Eastern Domain: 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.