If they (and you) read up on what travelers, missionaries, & ethnographers actually witnessed & recorded of recently contacted cultures, you will find that SoV & medium of wealth transfer long preceded record-keeping. See https://twitter.com/NickSzabo4/status/954225789129469952 … & refs in the linked articles.https://twitter.com/mikejcasey/status/981141578285805569 …
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sure, but no coins. They had huge amounts of gold and silver, all in the form of jewelry and larger artifacts. For everyday transactions they seem to have used these ledgers made of strings instead of a currency.
For SoV purposes it doesn't matter that their monetary metals were in some other form besides coins (as indeed most monetary metals were for most of their history).