The first part of this composite Indus character is the ideograph for the midpoint on a journey. In this instance, it should be read as 'on the way'. An extra area of road has been added on the right hand side. The right hand side takes precedence. Hence, Mahadevan's sign number 215 is an ideograph for the verb to pass.
Illustrative Text Reference:
Harappa: Seal: H-473 a: Sayid Ghulam Mustafa Shah and Asko Parpola, 1991: Corpus of Indus Seals and Inscriptions: Volume 2: Page 281: Collections in Pakistan: Suomalainen Tiedeakatemia.
Image Credit:
Indus Script Sign Number 215: Sign List of the Indus Script: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.