The key identifier for this pictograph is the two legs, because everyone knows that a human being normally walks on two legs. It is a simplified profile of someone walking, which is an ideograph for the noun person. This symbol shares a common root with the Chinese character rén 人.
Illustrative Text Reference:
Banawali: Seal: B-8 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 345: Collections in India: Suomalainen Tiedeakatemia.
Comments:
The inscription on B-8 is an excellent illustration of the importance of the motif on a seal. The markhor motif incorporates the Indus glyph for water. The water glyph is not horizontal, but is angled, so that under the Indus convention, the water flows from north to south. Hence, it is an ideograph for flowing water, in other words a river. When this symbol is read alongside the other two glyphs, the inscription on B-8 reads 'river carry people'. Thus, it can be conjectured that the seal's owner may have been a ferryman.
Mahadevan's sign numbers 1086 and 4693 may be variants of the same sign. It is not yet possible for me to say whether they each had a distinct meaning. I have listed them separately in this section with specific examples of their usages.
You should note that in inscription B-8, the people are moving/travelling on the river, whereas inscription M-856 merely refers to someone's occupation. The movement may be the distinction between the two forms of the symbol.
Image Credit:
Indus Script Sign Number 1086: List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.