It was these essays (The Visions of Sane Persons and Visualised Numerals), linked from the number form wikipedia article.

I think the cultural differences thing was just my conjecture, because the recorded number forms (as well as my own) often had turns at twelve and then subsequently at the decades, which led me to believe that they were probably based on the cadence of counting in English (one, two, three... ten, eleven, twelve, thirteen, fourteen... nineteen, twenty, twenty-one... twenty-nine, thirty, thirty-one, etc.). Whereas a Spanish speaker might be likely to develop a number form with a turn at 15 instead of 12, and a Chinese speaker to develop one with turns just at the decades.