The vertical line in this Indus sign can be likened to the Chinese character gǔn 丨. It is produced with a downward stroke.
Therefore, Mahadevan's sign number 1030 is an ideograph for something being dropped into a container. It is the verb to collect.
Illustrative Text Reference:
Harappa: Seal: H-24 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 173: Collections in India: Suomalainen Tiedeakatemia.
Image Credit:
Indus Script Sign Number 1030: List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.