This is a pictograph of a double-headed drum. Two packs portray the drum skins, and a horizontal line represents the drum shell.
Illustrative Text Reference:
Mohenjo-daro: Seal: M-326 d: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 82: Collections in India: Suomalainen Tiedeakatemia.
Comments:
Seal M-326 has text on five sides.
Packs would often have been wrapped in hides in order to protect the contents during transport. Therefore, the pack symbol is associated with hide, which is of course, what the drum skins were made of.
In the context of inscription M-326, Mahadevan's sign number 408 is the verb to signal.
Image Credit:
Indus Script Sign Number 408: Sign List of the Indus Script: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.