This is a pictograph of six counting rods. It is the numeral six.
Illustrative Text Reference:
Mohenjo-daro: Seal: M-678 a: Sayid Ghulam Mustafa Shah and Asko Parpola, 1991: Corpus of Indus Seals and Inscriptions: Volume 2: Page 35: Collections in Pakistan: Suomalainen Tiedeakatemia.
Comments:
You should note that this symbol only takes up half the height of the character area.
It is important to distinguish Mahadevan's sign number 108 from the three broken lines that represent maximum possible darkness, and hence, the noun winter.
Image Credit:
Indus Script Sign Number 108: Sign List of the Indus Script: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.