This is a pictograph of a measuring rod. There are three spaces which gives us three units of measurement.
Illustrative Text reference:
Mohenjo-daro: Seal: M-967 a: Sayid Ghulam Mustafa Shah and Asko Parpola, 1991: Corpus of Indus Seals and Inscriptions: Volume 2: Page 94: Collections in Pakistan: Suomalainen Tiedeakatemia.
Comment:
In inscription M-967, the measuring rod symbol is repeated. The second rod takes the generic meaning long. This usage of the symbol, makes it explicit that three units refers to the length and not the diameter of the pipe(s).
Image Credit:
Indus Script Sign Number 4320: List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.