This is a pictograph of a stockade fence. Hence, it is the noun pen. In this instance, there are three spaces between the fence stakes, which gives us a size three pen.
Illustrative Text Reference:
Harappa: Tablet in bas-relief: H-218 A: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 214: Collections in India: Suomalainen Tiedeakatemia.
Comment:
Tablet H-218 may have been a permit, for a cart with a specified load, to enter the city.
Image Credit:
Pen Size 3: Lynn Fawcett, 2018.