This symbol portrays a sundial. The line on the right is the gnomon, and the line on the left is the shadow cast on the dial. The diagonal line depicts movement, in other words the passage of time. Therefore, Mahadevan's sign number 3109 represents an unspecified period of time.
Illustrative Text Reference:
Mohenjo-daro: Seal: M-4 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 3: Collections in India: Suomalainen Tiedeakatemia.
Image Credit:
Indus Script Sign Number 3109: List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.