It is easy to see the natural progression from Indus sign number 86, the pictograph of a narrow track, to the other signs within this group.
For image credits, please see below.
Indus Script Signs with Numbers: Sign List of the Indus Script and List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General
Archaeological Survey of India.
Indus Script Signs without Numbers: Lynn Fawcett, 2017 - 2019.