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 three spaces between the vertical bars, which gives us a
size three cage.
Sample Text Reference:
Mohenjo-daro: Seal: M-34 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 18: Collections in India: Suomalainen Tiedeakatemia.