This is a pictograph of a floor area divided up by mats. It is an ideograph for the plural noun rooms or compartments.
Illustrative Text References:
Harappa: Seal: H-60 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 183: Collections in India: Suomalainen Tiedeakatemia.
Mohenjo-daro: Seal: M-1719 a: Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3,1: Page 28: New material, untraced objects, and
collections outside India and Pakistan: Suomalainen Tiedeakatemia.
This is Mahadevan's sign number 242, which I have redrawn to better reflect the original.