This ideograph is identical to an archaic version of the Chinese character xià 下. It is the adverb down or below.
Illustrative Text Reference:
Mohenjo-daro: Tablet: M-483 A: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 116: Collections in India: Suomalainen Tiedeakatemia.
(Unfortunately, the quality of the photograph in the Corpus is not very good. See also, Mahadevan's drawing: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: Appendix III: Illustrations of Field Symbols: Plate VII: Drawing Number 145: Page 813: The Director General Archaeological Survey of India.)
Comments:
There is only one extant example of this Indus sign.
In modern Chinese, xià refers not just to movement or position in physical space, but also to movement or position in time.
Image Credit:
Down: Lynn Fawcett, 2017.