This is a pictograph of a wooden staircase viewed in profile. The spaces between the newels and the spindles depict three steps. The steps descend from right to left. This symbol is a mirror image of the symbol for upstairs. It is therefore an ideograph for the adverb downstairs.
Illustrative Text Reference:
Mohenjo-daro: Seal: M-1680 a: Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3,1: Page 13: New material, untraced objects, and collections outside India and Pakistan: Suomalainen Tiedeakatemia.
Comment:
Indus inscription M-1680 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: List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.