The Indus Dictionary Project
The numeral twenty is comprised of two units of ten.
Harappa: Inscribed Pot: H-2263 C: Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3,1: Page 300: New material, untraced objects, and collections outside India and Pakistan: Suomalainen Tiedeakatemia.
This is Mahadevan's sign number 313, which I have redrawn to better reflect the original.
H-2263 is a large jar. It resembles a modern kvevri. It would probably have been set in the ground. Twenty is the size of the jar in Indus units. The number was inscribed on the shoulder of the jar, where it could easily be read from above.
Another interesting fact about H-2263 is that the symbol for return trip was etched in graffiti on the lip of the jar. This indicates that such jars were intended to be reused.
Numeral Twenty: Lynn Fawcett, 2018.