The Indus Dictionary Project
This is a pictograph of a furnace with vertical sides. The chimney and stoke holes are clearly visible.
Mohenjo-daro: Seal: M-644 a: Sayid Ghulam Mustafa Shah and Asko Parpola, 1991: Corpus of Indus Seals and Inscriptions: Volume 2: Page 23: Collections in Pakistan: Suomalainen Tiedeakatemia.
It is clear from the surviving pictographs that more than one type of furnace was used by the Indus Valley Civilisation.
It may be that the vertical sides in this symbol were meant to portray a kiln used for making bricks. The bricks would have been stacked prior to firing.
Furnace with Vertical Sides: Lynn Fawcett, 2017.