The Indus Dictionary Project
This is a pictograph of a snake. The downward strokes evoke snake strike action. Hence, this Indus sign is either the singular or the plural form of the noun snake.
Mohenjo-daro: Seal: M-68 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 30: Collections in India: Suomalainen Tiedeakatemia.
This is Mahadevan's sign number 2518, which I have redrawn to better reflect the original.
Snake: Lynn Fawcett, 2017.