Step 1: Recalling the Text:
The story builds its central mystery around small, easily-overlooked clues that hint the third level genuinely connects to 1894.
Step 2: Identifying the Detail:
The Galesburg fishing-resort notice is one such clue, its outdated details revealing that Sam's message had come from another time.
Final Answer:
It was an advertisement for a fishing resort in Galesburg — a place Charley found, on checking, had ceased to exist for over thirty years, deepening the story's mystery about the third level.