The numeral seventy is comprised of seven units of ten.
Illustrative Text Reference:
Mohenjo-daro: Copper object: M-2121 B: Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3,1: Page 128: New material, untraced objects, and
collections outside India and Pakistan: Suomalainen Tiedeakatemia.
This is Mahadevan's sign number 315, which I have redrawn to better reflect the original.
There is only one extant example of this Indus sign.