The Indus Dictionary Project
This ideograph is identical to an archaic version of the Chinese character shàng 上. It is the adverb up or above.
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.)
M-483 is the only extant Indus inscription with this sign.
Inscription M-483 reads: up, down, up, which I interpret as: to and fro.
Up: Lynn Fawcett, 2017.