The Indus Dictionary Project
This is a pictograph of a palisade fence. Hence, it is the noun pen. In this instance, there are three fence stakes, which gives us a size three pen.
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.
Tablet H-218 may have been a permit, for a cart with a specified load, to enter the city.
Pen Size 3: Lynn Fawcett, 2018.