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They were using Roman numerals. It's true that with Roman numerals and writing out the number using words, it's harder to forge. See https://m.reddit.com/r/AskHistorians/comments/49z4d6/why_did...

As explained in the Reddit link posted by 0942v8653, zero was not "banned", nor Arabic numerals, the 1299 Statuto dell'Arte del Cambio ( "Statute of the Art of Exchange" ) stated that the Arabic-like notation was forbidden for book-keeping to the members of the guild. (because the authorities were preoccupied by how easy to was to forge numbers). It is not unlike any technical norm (say from the Order of Engineer) stating (say) that SI unit of measures must be used, such prescription does not "ban" anything, they simply state how something specific has to be done, but has obviously effect only within the members. All the other people in Florence could use (and used) the new "positional" notation.

Perhaps CI? Or anyway a system similar to the Roman one where you essentially add different positive quantities?