The cryptic note penned by Abraham Lincoln identifies its recipient only as "my dear Sir" and has a small section carefully clipped out.

Who was he writing to and why was a key piece of information later removed so meticulously?

Historians believe they have unraveled the mystery and uncovered a bit of political intrigue in the process.

Researchers at the Papers of Abraham Lincoln project concluded Lincoln was writing to an ally to ask him to maintain a secret relationship with a political insider during the 1860 election campaign....