I doubt I will ever get to the bottom of it, but it has thrown up some interesting thoughts:
The rule of thirds has no basis. It appears to have come from poorly explained recipes, and people training without really understanding what they are doing.
(That said – a well made drink of thirds is really very tasty. Its just not a traditional cappuccino, and probably deserves a name of its own.)
The cappuccino is older than modern espresso (1948 I suppose).
Chocolate on top may well be traditional.
There is no way I am going to really understand the beverage unless I learn more about the history of milk consumption, particularly in around the turn of the 20th century.
The role of the Capuchin monks needs further exploration. What is it about the colour of their habits in particular that makes them so unique. I should really track down some sort of history of the order.
The paper I planned to write could easily end up as a book. It would be quite nice to incorporate all the sociology, food history, coffee history as well as some food science and some machine history to give a good overall picture. If someone has written this book then I am yet to find it!