I have heard about this for years. Just two weeks ago I had a conversation with an old friend about it. He is an interesting guy. He is obsessed with Baum and the Wizard of Oz. I brought up the allegory theory to him. He insists it's not true. He has researched Baum extensively and said that there is no evidence that he had any interest in the monetary debates of the day. He apparently has given lectures on the subject. I'm going to try to find these lectures to get more details.