The Indus Dictionary Project
This symbol may be a combination of the verb to join and a pictograph of the end of a walking stick. It may be related to the Chinese character pū 攴, which means to rap or tap. In Indus script, it is the verb to rap.
Mohenjo-daro: Seal: M-331 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 84: Collections in India: Suomalainen Tiedeakatemia.
Mahadevan may have assumed that M-331 was a tablet rather than a seal. The orientation of some of his Indus signs therefore differs from that of Joshi and Parpola.
For me, this Indus sign brings to mind, the rear view of someone rapping on a door with a stick, but that origin is pure speculation on my part.
Indus Script Sign Number 1471 (mirrored): List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.