This symbol is derived from a pictograph of bamboo or bark strips bound together. It is equivalent to the Chinese character cè 冊, meaning book or register. In modern Chinese, cè is the classifier (counter or measure word) for books, and means volume.
It should be remembered that in the past, a book was often a register used to record information. Hence, I infer that in the Indus script, this symbol is the noun register.
Illustrative Text Reference:
Two clay Sealings from Harappa: Photograph 1: https://www.harappa.com/blog/two-clay-sealings-harappa: Accessed: 18 September 2017.
Comment:
The illustrative text can be seen in a high quality photograph, on the Harappa website, 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 photograph is 'register string large'.
Image Credit:
Register: Lynn Fawcett, 2017.