What do Many Popes, Kings, Clocks, Dates, and Tables of Contents Have in Common?

Also, one method that works for many but not all of the numbers is to determine the smallest # of U.S. coins that would give a certain number of cents change. Like 15 cents is a dime and a nickel, so 2 coins. 8 cents is a nickel and 3 pennies, so 4 coins. 20 cents is 2 dimes. 6 cents is a nickel and a penny, so 2 coins. 50 cents is one coin. So you get the following, just like in the horse-hoof code:

6 -> 2
8 -> 4
15 -> 2
20 -> 2
50 -> 1