This man wears silk cocoons on his head. It indicates that his profession is something to do with silk. Hence, Mahadevan's sign number 5 is the noun silk merchant.
Illustrative Text Reference:
Mohenjo-daro: Seal: M-802 a: Sayid Ghulam Mustafa Shah and Asko Parpola, 1991: Corpus of Indus Seals and Inscriptions: Volume 2: Page 68: Collections in Pakistan: Suomalainen Tiedeakatemia.
Comments:
There is only one extant example of this Indus sign.
In reality, the headgear was most probably made from silk cloth.
Image Credit:
Indus Script Sign Number 5: Sign List of the Indus Script: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.