The Indus Dictionary Project
This is a pictograph of a stockade fence. Hence, it is the noun pen. In this instance, there are five spaces between the fence stakes, which gives us a size five pen.
Mohenjo-daro: Seal: M-140 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 45: Collections in India: Suomalainen Tiedeakatemia.
Indus Script Sign Number 2563: List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.