This is the symbol for area with the addition of a line which represents a third dimension. Hence, it is an ideograph for a cuboid. A cuboid was a measure of capacity. It can be likened to the Mesopotamian gur. In this instance, the cuboid is divided into seven Indus units.
Illustrative Text Reference:
Mohenjo-daro: Seal: M-1103 a: Sayid Ghulam Mustafa Shah and Asko Parpola, 1991: Corpus of Indus Seals and Inscriptions: Volume 2: Page 116: Collections in Pakistan: Suomalainen Tiedeakatemia.
Image Credit:
Capacity Seven Units: Lynn Fawcett, 2019.