Here are four independent sources verifying the authenticity of the letter posted HERE.

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From the book “Macedonia and Greece: The Struggle to Define a New Balkan Nation” By John Shea, 1997, page 60.

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From the book “Austria’s Wars of Emergence: 1683-1797″ By Michael Hochedlinger, 2003, page 162.

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From the book “Explaining Yugoslavia” By John B. Allcock, 2000, page 154.

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From the book “The Habsburg Monarchy, 1618-1815″ By Charles W. Ingrao, 1994, page 81.