Okay, I gotta give you more of my pennies.
The allegory makes sense, except 2 things. The Tin Woodman would more likely be Military, because he is the most violent of the gang. But he also cried when he stepped on a beetle and squashed it. He seems to defend the innocent, but also inheirts the Land of the Winkies who are good tin smiths. The Winkies, when sent by the Witch of the West to destroy the gang, have spears and weapons. So, military industry. The Lion would speak more of the power of the State, or the fear of the state. For he is terrifed of people, but also confused why people are scared of him. In fact, he's a bull in a china shop, to be honest.
Those aren't my interpretations - they're the ones proposed by Quentin Taylor, a historian. Apparently, the lion is also supposed to represent failed presidential candidate William Jennings Bryan, and the winged monkeys, Native Americans. Baum seems to have held contradictory views on Native Americans, but whether or not the winged monkeys actually represents them or not, is really debatable.
It seems more likely that all of these things say more about the people who proposed them than L. Frank Baum.