This rectangular shape represents the outline of a doorway. It is the noun doorway or the verb to enter.
Illustrative Text References:
Harappa: Seal: H-97 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 190: Collections in India: Suomalainen Tiedeakatemia.
Mohenjo-daro: Seal: M-331 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 84: Collections in India: Suomalainen Tiedeakatemia.
Comment:
Inscription H-97 is not a terribly good example because the seal is damaged, and there could be quite a lot of text missing. However, the inscription is of interest because Mahadevan's sign number 237 is repeated three times. A building with three doorways side by side was found by John Wheeler at Mohenjo-daro¹. Therefore, a known architectural feature is depicted in the script.
Image Credit:
Indus Script Sign Number 237: Sign List of the Indus Script: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.
Note:
1. Drawing: House 13, VS Area, Mohenjo-daro: John Wheeler, Excavator, 1922-1927: https://www.harappa.com/content/indus-house-1: Accessed: 15 December 2017.