This is the noun midpoint with the addition of a point of light in the left hand triangle. Reading from right to left: midday is followed by light. Hence, the period of light after midday is an ideograph for the noun afternoon.
Illustrative Text Reference:
Harappa: Seal: H-7 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 163: Collections in India: Suomalainen Tiedeakatemia.
Comment:
There is only one extant example of this Indus sign.
Image Credit:
Indus Script Sign Number 4008 (Afternoon): List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.