This is an ideograph for a group of animals or objects linked together with a rope or string. It was used as a classifier (counter or measure word) for equines. In a modern sense, it could depict a string of animals with each segment marked on the string representing a mule.
A second use for the classifier was with books or documents, which were of the strip type, held together by string.
Illustrative Text Reference:
Two clay Sealings from Harappa: Photograph 1: https://www.harappa.com/blog/two-clay-sealings-harappa: Accessed: 6 September 2016.
Comment:
The illustrative text can be seen, on the Harappa website, in a high quality photograph of a sealing that would have been attached to a large book or register. The literal reading of the three characters that can be clearly seen in the inscription is 'register string large'.
Image Credit:
Indus Script Sign Number 1006: List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.