This is a pictograph of a pen, to which bars have been added across the top. In other words, it is a cage. In this instance, there are two spaces between the vertical bars, which gives us a size
Illustrative Text Reference:
Mohenjo-daro: Seal: M-119 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 41: Collections in India: Suomalainen Tiedeakatemia.