So basically
2 commas = "Millions"
3 commas = "Billions"
4 commas = ... 2 commas = "Millions"
3 commas = "Billions"
4 commas = ... N-illion = (Size_of_Digit_Grouping)^(n-1)
Since the OP was saying groupings use to be by 6-digits, not 3-digits (i.e. "1,000") as seen today.