The Indus Dictionary Project
This symbol is water under a cover. The cover is the equivalent of the Chinese character mì 冖. Note how the water symbol has been trimmed. Only the central portion of the character is shown. The intention is to portray water constrained in a narrow channel. Hence, this Indus sign is an ideograph for the noun drain.
Mohenjo-daro: Seal: M-367 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 92: Collections in India: Suomalainen Tiedeakatemia.
There is only one extant example of this Indus sign.
Inscription M-367 is incomplete, however, the first three characters read drain in street, which suggests that Mahadevan's sign number 131 represents a covered street drain of the kind photographed by Munir Khan in Mohenjo-daro¹.
Indus Script Sign Number 131: Sign List of the Indus Script: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.
1. Munir Khan: Indus Valley Street with Covered Drain: Harappa.com: https://www.harappa.com/slide/indus-valley-street-covered-drain: Accessed: 11 April 2020.