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 six spaces between the vertical bars, which gives us a size six cage.
Illustrative Text Reference:
Mohenjo-daro: Seal: M-832 a: Sayid Ghulam Mustafa Shah and Asko Parpola, 1991: Corpus of Indus Seals and Inscriptions: Volume 2: Page 74: Collections in Pakistan: Suomalainen Tiedeakatemia.
Image Credit:
Cage Size 6: Lynn Fawcett, 2018.