I have stumbled upon this link on the web: http://www.languagesandnumbers.com/how-to-count-in-italian/en/ita/.
I states: "Numbers are grouped in words of three digits, with the specific rule that a space is added after the word for thousand if its multiplier is greater than one hundred and does not end with a double zero (e.g.: duemilatrecentoquarantacinque [2,345], seicentomiladue [600,002], settecentosessantacinquemila duecento [765,200])."
I cannot find any confirmation in any Italian source. Moreover, in some reports of Italian authorities, I always see such complex numbers written as a single word, regardless of the "double zero" presence.
So, is this rule a nonsense?