Step 1: Context:
In Jack Finney's 'The Third Level', Tom recounts to his psychiatrist friend Charley a story about a man named Sam who claimed to have found the mysterious third level of Grand Central Station and escaped into the past, to Galesburg, Illinois, in the year 1894.
Step 2: Answer:
Charley confirmed the story was true by showing Tom old, first-day-cover style letters that Sam (a stamp-collecting acquaintance of Charley's) had mailed from Galesburg in 1894, entrusting them to Charley to hold as an investment. One of these letters, addressed directly to Charley, revealed that Sam had indeed found the third level, successfully escaped to 1894 Galesburg, was happily settled there running a hay, feed and grain business, and invited Charley to join him if he ever found the third level too.