This composite character is the noun goods with the addition of a single counting rod (the numeral one). The numeral indicates the relative size of the shipment. Hence, Mahadevan's sign number 2115 is an ideograph for a small quantity of goods.
Illustrative Text Reference:
Mohenjo-daro: Seal: M-315 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 78: Collections in India: Suomalainen Tiedeakatemia.
Image Credit:
Indus Script Sign Number 2115: List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.