Here, I have redrawn Mahadevan's sign number 3108 because I don't think that the dimensions in the original are quite right. The symbol is a pictograph for the noun pit or well. It shares a
common root with the Chinese character jǐng 井.
Illustrative Text Reference:
Mohenjo-daro: Seal: M-68 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 30: Collections in India: Suomalainen Tiedeakatemia.