The Indus Dictionary Project
This is a pictograph of six counting rods. It is the numeral six.
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.
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 darkness, and hence, the noun winter.
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.