This is a pictograph of a crucible. In the base of the vessel is the ideograph for copper, which has been placed within a mountain shape. Hence, it represents a heap of copper ingots. The two vertical lines portray the air inlet from a bellows.
Illustrative Text Reference:
Harappa: Copper axe: H-1518 A: Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3,1: Page 205: New material, untraced objects, and collections outside India and Pakistan: Suomalainen Tiedeakatemia.
Comments:
You should note that in both Mahadevan and Parpola, sign number 4902 is shown upside down.
In the opinion of Kenoyer and Miller, there is little evidence to support the smelting of copper ore at Indus Civilisation sites¹. Therefore, the crucible portrayed in Mahadevan's sign number 4902 was probably used to melt copper ingots.
The axe has a broken edge, so it is not possible to say if inscription H-1518 is complete. However, the second character refers to the size of a floor area. Consequently, it is possible that the inscription referred to the copper workshop where the axe was made.
The ideograph for copper may be related to the position of the sun at sunrise and sunset when a copper coloured sky might be seen.
This would also explain the use of the same symbol to represent the planet Venus (the morning and evening star).
Image Credit:
Indus Script Sign Number 4902 (inverted): List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.
Note:
1. Jonathan Kenoyer and Heather Miller, 1999: Harrapa.com: Metal Technologies of the Indus Valley Tradition in Pakistan and Western India: Page 123: Originally published in: The Archaeometallurgy of the Asian Old World, ed. Vincent C. Pigott. University of Pennsylvania Museum Monograph 89. University Museum Symposium Series Volume VII. MASCA Research Papers in Science and Archaeology Volume 16. Philadelphia: University Museum Publications, University of Pennsylvania: https://www.harappa.com/sites/default/files/pdf/Kenoyer1999_Metal%20Technologies%20of%20the%20Indus%20Valley%20Tradition.pdf: Accessed: 20 April 2020.