This is a pictograph of a wooden staircase viewed in profile. The spaces between the newels and the spindles depict three steps. The steps ascend from right to left. The staircase, with three steps, is the only staircase symbol to be found with this orientation. It is, therefore, an ideograph for the adverb upstairs.
Illustrative Text Reference:
Mohenjo-daro: Seal: M-1992 a: Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3,1: Page 96: New material, untraced objects, and collections outside India and Pakistan: Suomalainen Tiedeakatemia.
Comment:
Indus inscription M-1992 is read from right to left. I have, therefore, assumed that the direction of travel on the staircase is from right to left.
Image Credit:
Indus Script Sign Number 4102 (mirrored): List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.