Hoax Letters, W.C. Fields, and a Giant Mystery

I have a family documents conundrum which just gets more and more interesting the more I learn. It began with a family letter from 1939 that couldn't be attached to any relatives, despite the salutation "Dear cousin". This spring, I posted the text of the letter on-line, and within a few weeks had been contacted by someone entirely unrelated who had a very similar letter.

Now I have discovered that W.C. Fields, the famous comedian, also had a letter like this, which has been posthumously credited to a niece.

I am baffled. The letter is very humorous, and I have to think it must be a hoax. But its origins are still mysterious. What is the history of "chain letters" and "spam" before the Internet? Could this have been some kind of propaganda?

Please read the whole story, and if you know more (or if you have a similar letter), please contact me!