Good question, and some anons speculated about that in the subsequent threads/breads. Of the various events discussed, a few anons proposed that 1909 was either the year income tax was introduced OR major changes to tax structure had taken place. That, to me, seemed the most plausible.

I could not, however, completely follow their train of thought because, unfortunately, all the images/infographics in the archived breads/threads are dead for me. I can’t open, much less see, them on my tablet. (My head hurts from trying, KEK.) A lot of info was shared via pics and graphics on the Q Boards so not to have access is a real bummer.

Anyhow, near as I can tell from their posts, 1909 seems to have been a year when the federal government had begun to change the way they operated for revenue. And, apparently, according to one anon, the actions of 1909 ultimately led to the creation of the Federal Reserve in 1913:

The National Monetary Commission was a study group created by the Aldrich-Vreeland Act of 1908. After the Panic of 1907, the Commission studied the banking laws of the United States, and the leading countries of Europe. The chairman of the Commission, Senator Nelson Aldrich, a Republican leader in the Senate, personally led a team of experts to major European capitals. They were stunned to discover how much more efficient the European financial system appeared to be and how much more important than the dollar were the pound, the franc and the mark in international trade. The Commission's reports and recommendations became one of the principal bases in the enactment of the Federal Reserve Act of 1913 which created the modern Federal Reserve system. THE FED WAS THE BEGINNING OF THE END